Cornell University Ergonomics Web

DEA3500: Ambient Environment: Illumination and Work

Many studies have addressed the problem of how much light is needed for optimal task performance.

Elton (1920) - compared output of silk weavers over 15 week period in winter time against the amount of daylight indoors over same period in an average year. Clear relationship between illuminance and output.

Weston (1920) -compared mean output for 20 linen weavers in October, November, and December, plus daylight inside weaving shed and plotted these against time of day (pm). Output decreased with decreased illuminance. As lighting became inadequate for the task to be clearly seen, performance declined.

Stenzel (1969) - Measured output from 12 people at a leather goods factory (bags, purses) over a four-year period in the middle of which the lighting was changed. 1957-1959 lighting was daylight and fluorescent at 350 lux. 1959-1969 little daylight and uniform fluorescent at 1000 lux. Found statistically significant improvement with increased illumination. Similar results have been found for various laboratory based task simulations.

Hawthorne Studies (Mayo) - True details of studies never published in journals. 1924 - Western Electric Co.

Experiment.1: Groups of employees in 3 departments inspecting small parts, assembling relays and wound coils. Baseline record of each person's output under daylight and artificial light. Then lighting intensity increased by 3, 6, 14, 23 fc in inspection department. In Inspection Dept. Rates of production "bobbed up and down without direct relation to amount of light". In Relay Assembly Dept. Lighting increased by 5, 12, 25, 44 fc and output increased. In Coil Winding Dept. Lighting increased by up to 46 fc and output increased but also increased when lighting decreased. So-called "Hawthorne Effect" BUT reanalysis of Hawthorne data shows that employees paid at piece rate increased output because of feedback on performance i.e. knowledge of results.

Experiment 2: Coil-winding Dept. Employees split into 2 equal-sized groups with comparable output records. Groups worked in separate buildings. One group was control (16-24 fc and daylight), one group had augmented lighting. (24, 46, 70 fc). 2 groups increased output at comparable rates. No effect of lighting.

Experiment 3: No daylight. Same two groups. One group - 10 fc constant (control) One group - 10 fc decreased to 3 fc (experimental group). "Efficiences of test and control group increased slowly but steadily." Until test group couldn't see what they were doing. Subsequent "informal" studies - workers liked brighter light but when bulbs replaced with identical ones workers commented like increased light! Light then decreased and then replaced with identical bulbs and workers said "'lesser' light was not so pleasant."

Visual acuity (VA) - VA is the angle subtended at the eye by the detail that can be discriminated on 50% of the occasions it is presented. Usually acuity is determined using black and white lines of equal thickness. However, in practice, acuity is determined by factors in addition to size of detail such as complexity of form, characteristics of viewing background, observer expectations etc.

Contrast

Contrast = C = | Lt - Lb / Lb | where Lt = target luminance, Lb = background luminance or sometimes C = Lmax - Lmin / L where Lmax = maximum luminance of a grating, Lmin = minimum luminance of a grating, L = average luminance.

Utilization coefficient (utilization factor)

This is the proportion of light that is reflected collectively by all the surfaces in a room.

e.g.
Reflection of surface - %Utilization
CeilingWallsFloorFurniture Coefficient %
65401228 29
85728550 57


p> In larger rooms the utilization coefficient is larger because less light is absorbed by surfaces. However, this means that in small rooms color is more critical (i.e. reflectance).

Luminance ratio

This is the ratio of the luminance of a given area e.g. the work area, to the surrounding area. Typically the ratio between task and other surfaces should not exceed 5:1 for office type tasks and 10:1 for industrial tasks.

Glare

This is produced by brightness in the field of vision that is sufficiently greater than the luminance to which the eye is adapted to cause annoyance, discomfort, decreased visual performance, and decreased visibility.

Types of glare:

Effect of direct glare on visual effectiveness

Glare source of 100 W incandescent lamp placed at different positions in relation to direct line of vision. As angle of glare source to direct line of vision decreases, visual effectiveness decreases.

(1 fc = lumen foot -2 = 10.76 lux).

Lighting Standards

Important point about lighting is that of itself it cannot produce work but poor lighting design can impair work. Need to match particular levels of illumination to task and situational characteristics.

Lighting Control Layout Design Considerations

A. Introduction

Adequacy and quality of illumination are important considerations for every type of work situation.

1. It has been estimated that an average data entry operator may undergo 33,000 head or eye movements per day.

2. The stress associated with these movements may be minimized with the proper attention to lighting, glare, illumination, contour sharpness, flicker and musculoskeletal constraints.

B. The human eye and vision

1. Eye anatomy The eyeball is contained in the orbit of the skull, surrounded and protected by a layer of fatty tissue. The eyeball is composed of the following layers:

a. Fibrous outer layer - the white of the eye (sclera).
b. Cornea (transparent layer that does not contain blood vessels).
c. Fibrous middle layer provides nourishment (blood supply) to the eye.
d. Retina or nervous layer. The retina is an extension of the optic nerve.
1. Rods are light sensitive.
2. Cones are color sensitive.
3. Tear ducts - the lacrimal gland secretes tears to wash the eye.
e. Eye muscles
1. Oculomotor muscles control eye movements.
2. Ciliary muscles control focusing. These muscles are active when looking at objects closer than 20 feet.
3. Constant close work (e.g. VDT data entry) requires nearly constant contraction of the ciliary muscles. Relaxation of these muscles is afforded by a change of focus, such as looking across the room momentarily or closing your eyes.
4. Iris muscles control amount of light penetrating the eye. The aperture formed by the iris is called the pupil.

C. Common eye problems

1. Nearsightedness (myopia) - The image of an object is focused in front of the retina.

2. Farsightedness (hyperopia) - The image of an object is focused behind the retina. Nearly 65% of the population suffers from one of the above refractive disorders.

3. Astigmatism - Irregular cornea curvature producing an image which is out of focus along one axis.

4. Cataracts - Opaque spots which form on top of the lens, obfuscating the light.
a. Cataracts are common with increasing age:
AGE % WITH CATARACTS
50-6443
75-8561
After data from Ederer, 1981.


b. However, only 2.7% of the 50-64 year olds and 31.6% of the 75-85 year olds with cataracts had a visual decrement of 20/30 or more.

5. Eye inflammation
a. A large number of diseases exist which produce:
1. eye inflammation
2. redness
3. discharge
4. blurred vision and other symptoms.

Bennet et al. (1977) - Found that there was a negative accelerating monotonic relationship between illumination level and task time for three industrial tasks.

Hughes and McNelis (1978) - Found a similar relationship between visual search times and illumination level for 2 clerical type tasks:
1. Find 3 digit numbers on worksheet with 420 handwritten numbers.
2. Letter-digit combinations in one column had to be found in second column.

Unusually there was little to suggest a leveling off in performance with increased illumination > 1500 lux.

Other factors e.g. age, have a significant effect on the relationship between illumination levels and performance.


Go to the next lecture