L03 - Lighting Research Issues

Berman, et. al. – Photopic luminance does not always predict perceived brightness

12 subjects in an almost uniformly white experimental chamber judged the perception of room brightness under photopic illuminances ranging from 30 cdm^-2 to 67cdm^-2.  Two different illuminants were compared which had different spectral compositions, but were colour matched.  Brightness judgements were often opposite to large differences in photopic luminance. These results are inconsistent with models of brightness perception that depend solely on cone receptors.  At the luminance levels considered here subjective evaluation of light intensity depends on both scotopic and photopic spectral contributions.  These results imply that aspects of the visual system operate mesopically under most interior lighting conditions.

Experimental method

·        12 volunteers – 20/20 vision

·        Spectrally flat surfaces in room

·        2 lamp combinations used

·        WWG – Warm white and Gold

·        R213 – Red and 213 – green

·        Adaptation to 36 cdm^-2 for 15 minutes

·        Random ordering of illuminants

·        Lamp outputs adjusted by dimming to yield similar chromaticities – achieved through subjective colour matching

Results

The photopically dimmer R213 was judged to be brighter in experiments 1 & 2 suggesting scotopic vision was the predominant factor in brightness judgement

However in experiment 3, WWG with a lower scotopic illuminance than R213 was chosen as brighter.  Therefore brightness is not solely influenced by scotopic and photopic response/stimulation.

Also, consider for large viewing fields that pupil size is a function of scotopic spectrum.  Hence rods affect pupil size.

Conclusion

Berman combines the photopic and scotopic responses to produce a new quantity, pupil lumens, to give a better photometric correlation with maintaining visual performance under different sources of light.  Berman suggest that if the photometric light quantity is reduced, thus reducing energy consumption, visual performance can be maintained by increasing the scotopic content of the source.  This relationship enhances the photometric effect of light sources having a higher photopic/scotopic ratio such as natural and simulated daylight sources.

Fotios, et. Al. – Visual perception under blue-rich tungsten sources

An experiment was carried out to assess the visual perception of a blue-glass tungsten lamp in comparison with an ordinary tungsten lamp.  The lamps were simultaneously presented in adjacent light booths to observers who were asked to balance the lights for visual equality by varying the supply voltage to the ordinary tungsten lamp.  Results show that on average the observers would prefer 35.8% less illuminance from the enhanced blue sources.  The performance of a visual task was maintained under the blue-glass lamps despite lower illuminance, and the majority of observers expressed preference for the luminous environment of the enhanced blue light for their workplaces.  Neither V(l) based photometry, Berman’s pupil lumens, nor Lynes’ photometric anomalies could account for the experimental results.

Introduction

V(l) photometry assumes fixed

·        Photopic levels of illuminance

·        2-3⁰ field size

·        neutral background

·        central fixation

In real life situations, these are not always applicable;

·        Berman – eyes operate mesopically in interior situations

·        Bartlett – field size which we notice luminances is 40⁰ band

·        Interiors vary in reflectances

·        Dynamic views are more common

Experimental method

·        40 volunteers

·        two colour matching booths

·        Blue-glass lamps held at fixed luminance.  Voltage control on ordinary tungsten lamps

·        Asked to adjust voltage until booths visually equal.

Results

·        Blue glass lamps needed 35% less light for equal perceived brightness

·        67% preferred blue glass lamp as working environment

·        No significant change in performance with change in spectrum

·        Performance increases with illuminance

·        Application of Berman’s and Lynes’ work does not fully predict the experimental results

Lynes, Daylight and photometric anomalies

Anomalies in the classical photometric system are outlined.  They are responsible for undervaluing daylight as an amenity by over 30%. Daylight is also undervalued as a source of task illumination.  Advantages of combining daylight with electric lighting are discussed.

Integrated daylight and electric lighting system assume a one-to-one tradeoff between electric light lumens and daylight lumens. Lynes suggests that the contribution of daylight lumens is underestimated by at least 30%. 

Standard photometry rests on two foundations,

·        The V(l) function, embodying the response of the visual system to lights of different wavelength.

·        Abney’s law which states that is two sources A & B each provide the same luminance, and two other sources C & D are similarly equal, then an additive mixture of A & C should look as bright as a mixture of B & D.

Neither is perfectly valid.

V(l) function

·        The Judd Correction – It is well known that the V(l) curve consistently underestimates the contribution of short-wave visible light.  This is compensated for by Judd’s Correction, which is a CIE endorsed modification to the V(l) curve.  Its effect tends to be small (1%)

·        Source size – V(l) is based on small field size, 2-3⁰, viewed centrally.  In this central foveal area, yellow macular pigmentation absorbs blue light, which would otherwise reach fovea.  This pigment is absent from the rest of the retina (no cones).  It has about 5% effect.

Abney’s law

·        Helmholtz-Kohlraush effect – This is where saturated colours tend to look brighter than neutrals having the same luminance.  Cowan & Ware give empirical proposals where they define a set of contours on the CIE chromaticity diagram which sets of colours which should look equally bright given equal photometric luminance.  We can show them numbered with the ratio of ‘luminance of average daylight/ matching luminance’ as a function of chromaticity.  This illustrates the well-known fact, that for a given photometric luminance, a higher colour temperature stimulus will look brighter than a ‘warm’ stimulus.  Applying this correction, we can see an underestimation by over 10%

·        Visual Clarity – It is also well known that, for a given illuminance, lamps having good colour rendering properties tend to make an interior brighter.  We consider the colour gamut of a source, and use an expression by Littlefair for luminance increment as a function of gamut area.  This expression gives a 15-25% underestimation depending on the colour gamut of the electric source that daylight is compared against.

·        Berman gives an alternative explanation for visual clarity, attributing it to a combination of photopic and scotopic luminances.  This allows for both visual clarity and Holmholtz-Kohlraush effect, giving an increment of 30%, which is comparable for the values given by the mainstream treatment.

Daylight and Electric Light

Taken at face value, we obtain an exchange rate of 130-140 electric lighting lumens for every 100 lumens of daylight.  This equates to about a 20-35% saving in energy. 

We assume that photometric discrepancies are cumulative.  It should also be emphasized that the discrepancy relates to subjective brightness, not necessarily to visual performance.  Therefore, when daylight is introduced into an artificially lit space, four benefits may be expected,

·        A helmholtz-kohlraush chromaticity bonus

·        A visual clarity bonus

·        A rod-response bonus

·        A reduction in flicker

Kuller – Melatonin, Cortisol EEG, ECG and subjective comfort in healthy humans : Impact of two fluorescent lamp types at two light intensities

Many field studies have shown benefits in using daylight simulating lamps in

·        Reducing stress levels

·        Increasing visual amenity

·        Behavioral aspects of children

·        Attendance levels

But some studies have also shown problems, where daylight lamps were,

·        Causing an increase in visual fatigue

·        Experienced as unpleasant and cool, especially during daylight hours.

Many of these apparent contradictions can be explained by the fact that these were all field trials, and did not control all experimental factors.  Kuller’s study attempt to reproduce some of these results under laboratory conditions.

Research shows that increased stimulus from lighting leads to increased reticular activity.  This can be measured, and used to measure response to light.

Experimental method

The study was carried out in a room with all daylight sealed out.  The following conditions were imposed

·        Subjects stayed in the room for an entire day

·        They carried out visual performance tasks throughout the day

·        Visual comfort readings were taken also

·        Blood samples, urine samples, EEG & ECG readings were taken throughout the day

Results

·        The daylight tubes caused more visual discomfort, especially at high luminance

·        Daylight tubes favoured visual acuity, even if the subjects did not think so

·        Fluorescent tubes at high luminances cause arousal of the central nervous system, especially if those tubes are of the ‘Daylight’ kind.

Begeman

This long-term study, by Phillips, looked at people in daylit window offices, and the amount of additional electric lighting that they required.  Results included,

·        People preferred to boost the daylight with quite high levels of electric lighting – often up to 800 lux

·        This seemed to be to compensate for brightness created by daylight

·        When daylight levels exceeded 2000 lux, the level of artificial light added increased

·        When daylight levels were below 2000 lux, electric component decreases

·        Mixed days were similar to clear days, except electric lighting levels are higher

·        Electric lighting levels varied over the day, with levels increased in the morning and late afternoon, with a dip in early afternoon.  This could be due to reticular stimulation.

·        VDT use was not impaired by high levels of illumination

·        Colour temperature was increased from 3300K to 4300K as illuminance increased.

·        Some individuals (Mr. BL & Mr. DL) prefer illumination levels either well above, or well below the average.

Lowe & Rowland – The art and science of lighting: A strategy for lighting design

This paper discusses the lighting design process and proposes an improved strategy, which provides a holistic approach including human response to the appearance of the whole visual environment.  In particular, it considers the aspects of visual function and amenity, integration with the architecture and energy efficiency.  It draws from previous work, experience, recent thinking and research studies.  Both the art and science aspects, together with their inter-relationships, need to be regarded in the process and a framework for design in proposed.  An all-embracing approach is necessary if both high quality in lighting and high energy efficiency are to be achieved.

Introduction

·        State of adaptation

·        Micro/macro views

·        Psychological response to sunlit day vs. overcast sky

·        Uplights vs. Downlighters

·        View out

Lighting for visual function

Consider with respect to the particular application,

·        Task illumination

·        Illuminance distributions in the task area

·        Luminance range and distribution within both micro and macro field with respect to adaptation state

·        Glare

·        Colour appearance

Lighting for visual amenity

Consider,

·        Composition of ‘visual lightness’ and ‘visual interest’

·        ‘Visual lightness’ relates to the illumination and the reflectances of surfaces, particularly the vertical surfaces that surround the field of view

·        ‘Visual interest’ relates to the composition of light and shade and the illuminance/luminance transition between the areas

·        40⁰ band work

Lighting and architectural integration

Consider,

·        The lighting appearance, including the pattern of light and the luminaires, needs to be a natural extension of the architecture

·        The transition of visual experience in terms of lighting from one space to another

·        The shape and form of individual rooms and the building as a whole; also architectural details.

·        The colour and surface finishes of the major surfaces

·        The daylighting performance

Lighting and energy efficiency, maintenance and lighting costs

Consider,

·        Use daylight wherever possible

·        Use lamps which are appropriate for the purpose, and have high efficacy

·        Use luminaires which have a high LOR and direct the light where it is required

·        Use electric light only when and where it is needed, particularly by the employment of lighting controls

·        Ensure good maintenance so light is not wasted

L03 (5). Emergency Lighting

Initial Considerations

·        Building Plans

·        Escape routes – a decision needs to be made when a route passes through an open area if it is to be considered an anti-panic area

·        Anti panic areas – areas greater than 80 m²

·        High risk task areas should be identified and normal lighting levels established

·        External illumination outside exit doors should be determined

·        Duration of the battery system required – normally 3 hours

·        Mode of operation of the luminaires – maintained/ non-maintained

·        Other areas which need illumination, but are not part of the escape route need to be determined i.e. lifts, plant rooms and toilet greater than 8 m²

·        Areas of low fire risk need to be identified if a central system is being used, the location of ventral batteries and cable runs should be established

·        Standby lighting requirements should be established if activities need to continue during a failure of the normal lighting supply.

·        Customer’s preferences and operating considerations

Legislation

1. Points of emphasis

We initially site luminaires at specific hazards and to highlight safety equipment and signs,

·        At each exit door

·        Near each staircase, so each flight receives direct light

·        At each change of floor level

·        To illuminate exit and safety signs

·        Near changes of direction

·        Near each intersection

·        Near each fire alarm call point

·        Near fire-fighting equipment

·        Outside each final exit, and close to it

2. Exit signs

Exit signs must be of the correct shape and size.  Max viewing distance is 200 x panel height

3. Essential areas

·        Lift cars, although only in exceptional circumstances will they be part of the escape route

·        Toilets with facilities exceeding 8m², and all facilities for the disabled

·        Escalators, to enable users to get off them

·        Motor generator, control or plant rooms require battery supplied emergency lighting to assist any maintenance or operating personnel in the event of failure

·        Covered car parks, the normal pedestrian routes should be supplied with non-maintained luminaires of at least one hour duration

4. Escape route lighting

When the points of emphasis have been located, fill in the escape routes to provide minimum illuminance along the routes.

·        BS 5266 Pt 1 1988 – This calls for a minimum of 0.2 lux at all points along the centre of an escape route, but unspecified higher levels to be used if old people or obstructions are present.

·        Draft European standard – This identifies the higher level as 1 lux for all risks.

5. Anti-panic open areas

The European draft encourages designs that do not use a few large luminaires.

·        BS 5266 Pt 1 1988 – 1 lux average over floor area

·        Draft European standard – 0.5 lux minimum anywhere on the floor

6. High risk task areas

·        BS 5266 Pt 1 1988 – No lighting levels specified

·        Draft European standard – 10% of the normal lighting level at the hazard, with a minimum of 15 lux.

L03(7). Daylighting for schools

Site analysis

The orientation and position of a building can affect the quality and quantity of light entering spaces, in addition to taking advantage of any pleasant view.  We consider siting in terms of the following factors,

·        Orientation

·        Solar Gain – heat generating activities should not be planned on that aside of a building where solar heat gain is likely to be a problem.

·        Overshadowing – Tall buildings and dense trees can screen from low angle sun

·        View

·        Light trespass – Night time lighting of playing areas can cause annoyance to neighbours. ‘Spill light’ and glare from floodlights should be minimized

Classroom lighting

We consider if a simple side-lit interior will give adequate daylighting.  If not, consider a rooflight or clerestory at the back of the classroom.

·        Use accent lighting on wall displays around the class room

·        To ensure maximum energy efficiency, consider

·        Optimize use of daylight - install time-switches to turn electric lighting off when not required

·        Install lighting controls in a logical way to ensure only the luminaires required will be switched on

·        Consider using HF gear to improve visual comfort and energy efficiency

·        Implement regular lighting maintenance programmes

Atrium lighting

·        The space is likely to be used for school meetings, dining, general socializing and private or group study

·        It may be landscaped, using suitable plants, and can form a focal centre

·        They will be predominantly lit by daylight, where available

·        They can provide a view from classrooms

·        They can increase daylight to the back of classrooms

There are considerations to be taken into account in terms of lighting levels,

·        It should not be so high that adjoining spaces feel under-lit

·        In order to achieve a satisfactory distribution of light, especially in the lower spaces, the majority of spaces should be of higher reflectance

·        The orientation of the atrium is important in terms of sun and sky glare as well as solar gain

·        Clear glass is recommended in order to give a view out

·        Provision of cleaning and maintenance is necessary, since these spaces are often two stories or greater in height

Some of the main factors to be considered are,

·        Sky glare

·        Sunlight glare and solar gain

·        Supplementation of daylight to adjoining spaces

·        Visual contact with exterior

Circulation areas

The circulation routes through a school are the main arteries taking pupils, staff and visitors from the main gate through to the various particular areas.  They need to be functional in that people need to find their way easily and safely, even when unfamiliar with it.  They also need to be visually stimulating.  Finally they need to be provided with means of escape and this may require emergency lighting.

Exterior circulation

During daytime the route to the main entrance will usually be obvious.  This will be due to the architectural treatment and the site organization.  At night-time, things will be different,

·        The main gate may need to be identified, perhaps with an illuminated sign

·        A high-lit area may be needed around the gate

·        This area needs to be linked visually to the main entrance of the school

·        The important thing is to light the pavement and road surfaces