L05 – Lighting Practice

1. Overview

2. Lighting Legislation I – Lighting related legislation

European and International Legislation

Current activities in European Standardization (From Lecture 5)

Construction, Design & Management Regulations (CDM)

Building Regulations, Part L

Construction and Safety of fittings (BS 4533)

3. Lighting Legislation II – CIBSE Code

Introduction to the code

Lighting recommendation for specific types of interior and activity

Maintained Illuminance

Modifying factors flowchart

Examples

4. Lighting Legislation III – CIBSE Lighting Guides

Lighting for offices

The industrial Environment

Hospitals and healthcare

Museum and art gallery guide

5. Design and construction issues I – Luminaires

Luminaire design and construction

Luminaire Design

Luminaire Testing

Luminaire Photometry

6. Design and construction issues II – Utilization factors

Calculation of utilization factors

7. Design and construction issues III – Daylight

Designing for daylight

9. Lighting Design in practice I –Lighting Application Studies [gd]

10. Lighting Design II – Designed appearance

11. Lighting Design in practice III – Case Studies

L05(2). Lighting Legislation I – Lighting related legislation

European and International Legislation

There are a wide range of legislation that a practicing lighting designer needs to be aware of, at the national, European and international level.

Standards bodies

International

·        IEC – responsible for electrical and mechanical safety standards

·        ISO – non-electrical standards e.g. building and ergonomic issues; daylighting

·        CIE – Art & Science of lighting – daylighting, electric lighting, photometry, colour, photo-biology, vision systems

At the European level, there are bodies that duplicate some of those at the international level,

·        CENELEC « IEC

·        CEN « ISO

·        CIE also works at this level

All of these bodies produce standards documents, attempting to harmonize standards across Europe. An EN standard is an agreed technical text, which carries with it the strict obligation to be implemented as a national standard.  A HD standard shows technical equivalence throughout Europe (??).

There are several EC directives that are relevant for lighting related work,

·        Workplace – This is embodied in the Health & Safety 1992 regulations.  It covers,

·        Daylight levels

·        Maintenance issues of glazing systems

·        Suitable and sufficient lux levels in the workplace

·        Advises that as far as possible, natural lighting should be used

·        Suitable & sufficient emergency lighting should be present

·        Use of Work Equipment – this discusses things that are used by people in the workplace, i.e. luminaires, which are covered by BS4533 – Luminaires.  In summary, it states that luminaires should be safe and healthy

·        Display Screen Equipment – This is covered in Health & Safety DSE 1992. It states that employers need to do risk assessments for health & safety.  It gives minimum standards for disturbing reflections.  It is codified in CIBSE LG3.  This provides limits for luminaire angular cutoff for areas of constant/ intermittent/ occasional DSE use.  This takes into account age and type of equipment (white on black, or black on white).  It states the main areas that cause problems are

·        High luminance reflections in screen

·        Static imbalance in far field

·        Dynamic imbalance in near field

It is the employers’ responsibility to consider working patterns and to provide

·        Adequate screen technology

·        Mini-breaks to give visual rest

·        Eyesight tests for employees

·        Safety signs at work – this covers self-illuminated emergency signs

·        Machine safety – localized lighting fitted to machines e.g. lathes.  Must provide independent task illuminance from environment.

·        Constructions products (CE mark) -  It is an offence to sell non-conforming equipment within the EC

·        Electromagnetic compatibility – Luminaires should not suffer interference or interfere with other equipment

OPUS lists all relevant standards.

Current activities in European Standardization (From Lecture 5)

·        Voltage harmonization

·        Pre 1995 – 240V ±  6%

·        Jan 1995 – 230V + 10% / - 6%

·        By 2003 – 230 ± 10%

·        No change in the supply voltage

·        Voltage harmonization agreed by DTI against wishes of lighting industry who cited safety problems if no change in supply voltage

·        Testing of Lighting products

·        Products marked 240V – tested at 240V ± 6%

·        Products marked 230V – tested at 230V ± 6%

·        UK input to standards w.r.t. luminaire testig based on ±6%, so UK had better safety than ±10% countries.  Importing 230V tested products can therefore give problems

·        All UK products now to be marked 230V

·        GLS Lamps

·        230V lamps on 240V

·        life reduced from 1000 hr to 560 hr

·        temperature increase of 5%

·        increase in accidents while chaging lamps

·        increased fire risk

·        Standard GLS lamps cause the worst problems if 230V lamps used on 240V supply

·        Increase in accident rate

·        Market penetration 1% - Deaths 0, Hospital treated injuries 26, fire brigade call-outs 6

·        Market penetration 15% - Deaths 1, Hospital treated injuries 400, fire brigade call-outs 100

·        Based on ERA study

Construction, Design & Management Regulations (CDM)

The CDM regulations plan, design and manage health and safety at construction and post-construction stage.  It covers all design work, and all construction work which last for over 30 days.

Principal Parties

The main parties in CDM are

·        Client

·        Designer

·        Principal Contractor(s)

·        Self Employed Contractors

·        Planning Supervisor

Each must do two things,

·        Health & Safety Plan – This takes input from the designer and contractor

·        Health & Safety File – This consists of a logbook which is handed over to the client.  It contains all the risk assessments and procedures developed during the build process.

The client appoints Planning Supervisor who

·        Controls designer, re. Health & Safety

·        Responsible for creating plan and file

·        Seeks info from contractor ( what was built)

The responsibilities of the designer include

·        Site analysis to identify main hazards during construction, maintenance and disposal e.g. mercury lamps

·        Where you can avoid hazards, come up with a ‘method statement’ in the form of a procedure which then goes into the logbook.

The designer may need to have chartered status, or be supervised by one.

Building Regulations, Part L

Part L is concerned with the conservation of Fuel & Power.  It states that a scheme must be designed and constructed to use no more than reasonable power and fuel.  It also must provide provision for reasonable control.  This applies to new build and change of use.

There are two ways to comply with part L,

·        Either 95% of installed lighting circuit watts of lamp types listed

·        SON

·        MK

·        Induction

·        Tubular Fluorescent (25mm diameter with HF ballast only)

·        Compact Fluorescent (all ratings > 11W)

OR

·        The lighting designer must demonstrate that the average efficacy of the installation > 50 lumens per circuit watt

Exemptions are given in certain cases – Exterior, Display, Domestic, Emergency Lighting.  Judgement is required in Churches, Cinemas, and other low use buildings.  If you work to the CIBSE Code recommendations, you will satisfy Part L.

Construction and Safety of fittings (BS 4533)

There is an agreed international document for Luminaire specification produced by the IEC, IEC 598 Luminaires.  This provides much of the basis for the British Standard BS4533 Luminaires. The corresponding European Standard is EN60 598-1.

The main functions of the luminaire are:

·        To redirect light from the bare lamp in the preferred direction with the minimum of loss

·        To reduce glare from the source

·        To be acceptable or contributory in appearance

Other important aspects are,

·        Lamp Protection – The lamp must be mechanically supported and protected

·        Electrical safety – The lampholder, control gear and associated wiring must be protected and supported

·        Heat dissipation – Heat from the lamps and control gear must be conducted away from heat-sensitive parts and surfaces near the luminaire

It is unlawful to make, or hold in stock, or offer for sale any electrical apparatus that is unsafe.  With regard to luminaires, BS4533 is accepted by HM Government as a “safety” specification.  Manufacturers can give assurance that luminaires are safe by marking them with the BSI safety mark. 

Luminaire Markings

Certain information is required by BS4533 to be marked onto luminaires,

·        Mark of Origin

·        Rated Voltage(V) (For GLS, no mark unless different from 250V)

·        Rated max ambient temp, if other than 25⁰C

·        Class of luminaire

·        0 – luminaire relies on basic illumination (not permitted in UK)

·        I – Functional insulation throughout with earthing terminal or earthing cable and plug

·        II - Double and/or reinforced insulation

·        III - luminaire protects against electric shock by running on a supply at safety extra low voltage (< 50V)

·        IP number (ingress against foreign bodies/ingress against moisture)

·        Makers model number

·        Rated wattage(W) of the lamps

·        Classification of the material of the supporting surface

·        Non-combustible/ no ballast, flammable surface/ ballast, flammable surface

·        Information concerning special lamps

·        Symbol, where luminaires use lamps similar in shape to ‘coolbeam’, where this might impair safety

·        Terminations should be clearly marked earth and live

·        Minimum distance of spotlight from lit objects

Luminaire characteristics (CIBSE pp. 75-83)

1.      Mounting position – recessed, surface, pendant

2.      Polar curve

3.      SHR NOM

4.      Utilization factor

5.      Glare index

6.      Room surface brightness

L05 (3). Lighting Legislation II – CIBSE Code

The CIBSE Code for Interior Lighting is aimed at

a)     Specifying the lighting conditions appropriate for a wide range of interiors

b)     Offering guidance on design methods for obtaining those conditions

It is organized in six parts,

Part 1.This summarizes the effect of lighting conditions on the performance of tasks, the appearance of the interior and the comfort of the occupants

Part 2.Recommendations of maintained illuminance and other design criteria, both specific and general, which are suitable for a large number of interior and exterior applications

Part 3.Describes the properties of interior lighting equipment which is generally available

Part 4.Sets out suitable design procedures

Part 5.Appendices

Part 6.Glossary, Bibliography and references

Recommendations for daylight

In most types of buildings, users prefer rooms to have a daylit appearance during daytime hours.  This appearance can be achieved, even if there is a significant amount of daytime electric lighting by ensuring that changing brightness of daylight is clearly noticeable of walls and other interior surfaces.  It is also necessary to achieve sufficiently bright interior surfaces to avoid glare from contrast with the sky.

Daylight for general room lighting

·        If daylight is not normally used during daytime hours, the average daylight factor should be not less than 5%.  The internal reflections and positions of windows should be such that the inter-reflected light is strong and even.  When the shape of the room causes the daylight distribution to be uneven, supplemental electric lighting may be required.

·        If daylight is to be used during daytime the average daylight factor should not be less than 2%. In a room with less than 2% average daylight factor, the appearance will be that of an electrically lit interior.

·        When daylight alone provides the task illumination, the illuminance should not fall below that given in the specific recommendations for the interior type.  The uniformity within the immediate task area should be similar to that provided by electric lighting although there may be variation in daylight in different part of the interior.

·        When there is a significant amount of daylight, electric lighting may be needed to reduce the contrast between internal surfaces and the exterior view.  It needs to fall on the walls and other surroundings of the window opening.  The brighter the view, the higher the required surrounding luminance.

·        If electric lighting is required to increase the illuminance in areas distant from the window, the average working plane illuminance due to electric lighting should not be less than 300 lux.  If lower levels are used there may be noticeable contrast between areas near windows and other parts of the room, causing an impression of gloominess or harshness.

·        In general room lighting, apparent discrepancies between the colour of electric lighting and daylight can be reduced by using lamps of intermediate colour temperature (3300-5300K) and screening lamps from the view of the occupants, using opaque louvres rather than translucent diffusers.

General recommendations for electric lighting

These recommendations form the foundation for the specifications and design of all electric lighting to ensure visual comfort and satisfaction.

Illuminance

The recommendations given for specific applications are consistent with the rule that working spaces, which are to be occupied for long periods, should have a maintained illuminance of not less than 200 lux on the working plane. We assume the people who are working have normal vision.  If a significant number of people occupying the space have some degree of visula impairment, the maintained illumnance could be increased.  The recommendations generally assume a viewer age of 40-50 years.

Illuminance Variation

For the task area, and the immediate surround, uniformity is important.  If the precise size of the task area is not known, calculations can be based on an area measuring 0.5m x 0.5m located immediately in front of the observer at the edge of the desk or working surface.

 

It is recommended that the uniformity of illuminance (minimum to average illuminance) over any task area and immediate surround should not be less than 0.8.

 

We must try and avoid excessive variation of horizontal illuminance across the space: the diversity of illuminance expressed as the ratio of the maximum illuminance to the minimum illuminance at any point in the ‘core area’ must not exceed 5:1.  The core area is that area of the working plane having a boundary 0.5m from the walls.  Installations lit by ceiling-mounted arrays designed by the ‘lumen method’ following the usual layout and spacing criteria will normally satisfy the uniformity requirements.

In a localized or local lighting scheme, the normal design method is to establish the highest recommended task illuminance, and then to set the ‘ambient’ level at one thrid of this value or to the requirement of the non-task areas (whichever is greater).  The illuminance at the task is then ‘topped up’ with localized or local lighting to the apropriate task level.

Luminance and illuminance ratios

Luminance differences may be specified or measured in terms of the ratio between one luminance and another.  Suggested targets are task-to-immediate surround 3:1, and task-to-general background 10:1.

Ceilings

The recommendation for general lighting with a predominantly downward distribution is for the ratio of average illuminance on the ceiling to the average illuminance on the horizontal working plane to be within the range 0.3 to 0.9.

In general the ceiling cavity reflectance should be as high as possible, at least 0.6.

For indirect lighting, the average luminance of all surfaces forming the ceiling cavity should be not more than 500 cd/m².  However small areas of luminance up to 1500 cd/m² will generally be acceptable, provided sharp changes from low to high luminance are avoided.

Walls

Higher reflectance of wall and partition surfaces will increase the perception of lightness in the interior.  Walls with windows are a particular case.  The walls surrounding a window should have a reflectance not less than 0.6 in order to reduce contrast.

The ratio of the average illuminance on the walls to the average illuminance on the horizontal working plane is related to the average vertical plane illuminance throughout the space.  This has been shown to give good correlation with visual satisfaction for office lighting.  The recommendation is for the ratio of the average illuminance on any wall or major partition surfaces to the average illuminance on the horizontal working plane to be within the range 0.5 to 0.8. 

In general the effective reflectance of the principal walls should be between 0.3 and 0.7.

 

Floor and working plane

The reflectance of the floor cavity plays an important part in visual appearance of a room.  With most lighting installations a proportion of the light on the ceiling will have been reflected off the floor.  Low reflectance bench and desk tops should be avioded since these surfaces have a major effect on effective floor cavity reflectance.

In general it is undesirable for the average floor cavity reflectance to exceed 0.40 or fall below 0.20.  The reflectance of the area surrounding the task should not be less than one third of the task itself.  In the case of office tasks involving white paper this will require desk tops to have a reflectance of at least 0.30.

Colour appearance

Some general rules to help with the selection of lamp source colour are,

·        For rooms lit to an illumination of 300 lux or less, a warm or intermediate colour temperature is preferred; cold apparent colour lamps tending to give rooms a gloomy appearance at lower illuminances

·        Where it is desirable to blend with daylight, intermediate CCT sources should be used

·        Different colour lamps should not be haphazardly used in the same room

Colour rendering

Where work involving accurate colour judgement is required, electric light sources with high CIE colour rendering indices (Groups 1A or 1B) should be used.  In general, light sources with good colour rendering properties (Group 1B) make objects appear more colourful than do those with medium colour rendering properties (Group 2, 3 and 4).

Modelling & Emphasis

Illumination which falls on an object from all directions, enables the object to be seen, but does not reveal much of the form or texture because there are few, if any shadows.  The relationship between the intensity of the directional lighting and the diffuse component is known as the vector/scalar ratio.  A vector/scalar ratio of 1.2 to 1.8 will prove satisfactory in normal general lighting conditions where perception of faces is important.  Under such conditions facial modelling will usually appear balanced and natural.

Display lighting tends to call for greater impact and emphasis.  The Display Iluminance Ratio (DIR) is that between the general horizontal plane illuminance and the value of local illuminance in the plane of the object to be displayed.  Greater degrees of emphasis are likely to require lower vaues of general diffused lighting to avoid the need for excessive values of local display illuminance.

Energy efficiency

Energy efficiency can be achieved in two ways

·        By using the most efficient lighting equipment

·        By using effective controls so that lighting is not in use when it is not needed

The recommendations state that an average initial circuit luminous efficacy of at least 65 lm/W for the fixed lighting equipment should be achieved.

Recommendations for specific types of interior and activity

These provide

·        Quantitative guidance on the maintained illuminance, and limiting glare index for a wide range of interiors and/or activities.

·        Qualititative advice on the aspects of the interior/activity which should influence the selection of lighting equipment and design of the lighting installation

·        Notification of statutory and advisory documents relevant to each interior/activity

Maintained Illuminance

The maintained illuminance, E_m, is the average illuminance over the reference surface at the time maintenance has to be carried out by replacing lamps and/or cleaning the equipment and room surfaces.

The Design Maintained Illuminance is the illuminance to which the interior should be lit after modifying factors have been taken into account departures from the assumed typical conditions,

·        The visual demands of the task

·        The duration of the work

·        The consequences of any erros

·        Energy considerations

5. Design and construction issues I – Luminaires

Luminaire design and construction

In ‘Design for the Real World’, V. Papanek describes a product design methodology which balances practical considerations with aesthetic ones.

We can apply these criteria to the design of a luminaire by asking the following questions,

·        Need – Do we need a new it?

·        Use – How is it to be used?

·        Aesthetics – Is it beautiful?

·        Materials, Tools, Processes – How do they interact?

·        Telesis – Which culture gave rise to it?

·        Association – What does it remind you of?

Luminaire Design

The function of the luminaire is:

·        To redirect light from the bare lamp in the preferred direction with the minimum of loss

·        To reduce glare from the source

·        To be acceptable or contributory in appearance

Other important aspects are,

·        Lamp Protection – The lamp must be mechanically supported and protected

·        Electrical safety – The lampholder, control gear and associated wiring must be protected and supported

·        Heat dissipation – Heat from the lamps and control gear must be conducted away from heat-sensitive parts and surfaces near the luminaire

·        Finishing – Attention must be paid to the protection of the luminaire finish to corrosion and degradation

·        Maintenance – Ease of cleaning and relamping

As a luminaire designer, one must appreciate the range of materials and surface finishes it is possible to use in a luminaire,

·        Metals

·        Ferrous – sheet steel

·        Non-ferrous – Aluminium alloys

·        Finishes

·        Stove enameling

·        Electrostatic spraying

·        Dry powder spraying

·        Plastics

·        Thermoplastic - Polystyrene, Acrylic, Polycarbonate, PVC, ABS, Nylon, PBT, Polypropylene

·        Thermosetting – Urea/Phenolic, GRP

These all have different properties,

Material	Melt Point (oC)	Operating Temp(0C)	Flammability
Nylon 66	260	-	Self extinguish
Acrylic	160-180	-	Flammable
General Purpose PVC	-	70	Unstressed
High Temp PVC	-	105
High Temp EVA	-	140
Silicone Rubber	-	200
PTFE	-	260
Polycarbonate	-	-90 -> +135	-

Reflectors

Diffusers

Material (all 3mm)	Transmittance (%)
Flint glass	92
Clear Acrylic	92
Opal Acrylic	50 - 80
Polystyrene	92
Polycarbonate	88

Luminaire Testing

Electrical Safety

·        Test finger – no live parts touched accidentally

·        Check creepage and clearance distances

·        Insulation resistance

·        Electric strength – high voltage flash test

Mechanical Safety

·        Fixing and suspension must be sufficiently strong to withstand overload conditions

·        Nut and screw fixings tested using torque tools

·        Corrosion testing in saline atmospheres

·        Impact testing by impact hammer ( or large prison officers )

Thermal Safety

·        Temperature endurance tests – Subject the luminaire to on/off switching cycles for seven days in ambient temperature, +10 degrees above ambient and at over voltage

·        Temperate tests take place in draught free enclosures using thermocouples placed at relevant positions with luminaires operating at 110% voltage.

·        No point on external surface should be hot enough to burn in normal use

·        Components should be heat resistance – each given a max operating temperature

·        Mounting position is important – distance from adjacent surfaces?

Other factors are important,

·        Marking of luminaires

·        Ease of maintenance

·        Catches and enclosures of fittings

·        Ease of cleaning

·        Ease of relamping

Luminaire Photometry

BS 5225 gives details of measurement techniques for obtaining photometric data for luminaires, including laboratory conditions, and procedures for simple luminance and illuminance measures.

A goniophotometer is used to measure the intensity distribution of a luminaire.  It uses a photocell which has a spectral sensitivity which corresponds to the V(l) curve.  We use calibrated lamps, which have been run for 100 hours to allow them time to stabilize.

Certain characteristics are required for the goniophotometer to produce accurate results,

·        Minimize stray light by light baffles

·        Control air temperature

·        Avoid errors  due to

·        Non-linear photocell

·        Voltage fluctuations

·        Mirror sagging

·        Mirror reflectance

The goniophotometer maintains the luminaire in the desired attitude.  You measure in the position of luminaire use.  To achieve the desired optical path length, mirrors are often used; the path should be at least five times the largest dimension of the light fitting.  The mirrors should be optically flat, and have a uniform surface finish.

Readings are taken in the azimuth and altitude of the luminaire.  In a luminaire that is nominally symmetrical, azimuthal readings are taken every 40⁰.  For all other luminaires, including asymmetrical luminaires, readings are taken every 30⁰.  In altitude, readings are taken every 10⁰, in the centre of the spacing e.g. 5,15,25…  If the luminaire  is light concentrating, take measurements every 5⁰.

6. Design and construction issues II – Utilization factors

CIBSE TM5 – Calculation & Use of utilization factors (1980) is concerned with the production of taking raw photometric data in terms of uncalibrated intensity distributions of luminaires and converting that into information which can be utilized directly in lighting calculations, in the form of utilization factor tables.

Certain assumptions are made in TM5,

·        Rooms are square (rectangular rooms of same room index are fine, provided L:W < 4:1)

·        Room surfaces are uniformly diffuse

·        Luminaires are in square array with half-spacing to perimeter

A non-rectangular room can be broken down into a series of adjoining squares rooms.

OCalculation of utilization factors

The procedure can be broken down into a series of discrete steps, each of which TM5 provides a worksheet for,

·        Calibrate of raw photometric data

·        Determine spacing of luminaires on which to base the UF table

·        Calculate proportion of bare lamp flux received directly by surfaces of room for each Room Index

·        Find the contribution from inter-reflected light for each set of room reflectances

The calculation of Zonal Flux and Light Output Ratio (Worksheets 1a & 1b)

We assume that the luminaire has been photometrically tested in accordance with BS5225: Part I.  A table of intensities will be available from this measurement.  For symmetric luminaires (usually incandescent), we require azimuth readings every 45⁰.  For all other luminaries, we require readings every 30⁰.

In order to calculate Zonal Flux, e only need readings at 5⁰,15⁰,25⁰, etc. Values at 10⁰, 20⁰, … 90⁰ are required for drawing polar curves, and calculating uniformity.

The results of this worksheet are,

·        Scale Factor ( ratio by which uncalibrated results must be multiplied to calibrate them per 1000 lamp lumens)

·        ULOR, DLOR and LOR

·        Calibrated axial and transverse intensity distributions

·        Zonal flux ( i.e. amount of lamp flux falling in each 10⁰ axial band in all directions from the luminaire

The selection of Spacing to Mounting Height Ratio

The uniformity ratio is defined as the minimum to average illuminance over the task area.  The code recommends this should not be less than 0.8.  The UF tables should be based on a spacing to height ratio (SHR) close to the maximum spacing to mounting height ratio at which this uniformity is achieved.  This will enable the most economical lighting installation to be planned.  In practice, the spacing may be closer to avoid shadowing or to increase the illumination.

Two simplifications are made for the purposes of the calculation,

·        A standard layout of 16 luminaires is adopted

·        The effects of inter-reflected light are ignored

The first simplification has little effect since adding more luminaires only marginally affects the illuminance at the centre.  The second simplification tends to slightly underestimate the uniformity that will be achieved, since inter-reflected light is more evenly spread than direct light.

A uniformity of 0.8 is normally achieved over the central region if the ratio of the minimum to maximum illuminance is greater than 0.7.

Two spacing to height ratios are calculated,

·        SHR MAX – the widest spacing at which a ratio of minimum to maximum illuminance greater than 0.7 is achieved

·        SHR NOM – the greatest value of SHR in the preferred series 0.25, 0.5, 0.75, etc. that will achieve this criterion.  Utilization factors are based on this value.

Distribution Factors

The distribution factor is defined as the fraction of bare lamp flux that reaches the reference surface directly (without inter-reflections).  For the purpose of the calculation, the room is considered to consist of only three surfaces, the horizontal reference plane, the luminaire plane and the walls in between these two.

The space above the luminaires (for suspended luminaires) and the space below the horizontal reference plane are designated the ceiling cavity and the floor cavity respectively.

The distribution factor DF(F) for the floor cavity is the proportion of bare lamp flux that reaches the floor cavity directly.  Similarly for DF(C) and DF(W).  To calculate the distribution factors, we use the zonal multipliers, which give the fraction of flux in each 10⁰ band.

Utilization Factors

We now have the amount of flux reaching the surfaces directly.  Now we calculate the inter-reflected component by using transfer factors.  The Transfer factor TF(S1,S2) from one surface S1 to another surface S2 is the ratio of the total flux falling on S2 as a result of the flux illuminating S1, to the direct flux on surface S1.  The Transfer factors from the ceiling, walls and floor onto the reference surface for a range of room indices and surface reflectance are given in TM5.  The Utilization Factor UF(F) is given as follows

There is a standard form for the publication of utilization factor data, which is shown overleaf.