15.5 Illumination, Lighting Design, and Visual Performance

Illumination, Lighting Design, and Visual Performance

The BGC subject area definition for non-ionizing radiation ends with "ultraviolet, visible, infrared radiation, and illumination." Lighting is the only physical agent in industrial hygiene where the goal is not to minimise exposure but to optimise it: too little light and too much light both degrade performance, and glare degrades it at any level.

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1. The Four Photometric Quantities

Confusing these is the most common source of lost points on lighting items.

Two relationships you should be able to use without hesitation:

$1\ \text{foot-candle} = 10.76\ \text{lux} \qquad (\text{1 fc} \approx 10\ \text{lx for rough work})$

Illuminance is what a light meter measures at the task. Luminance is what determines perceived brightness and glare, and it depends on both the illuminance and the reflectance of the surface:

$L \approx \frac{E \times \rho}{\pi} \quad \text{(for a diffuse surface, } \rho = \text{reflectance)}$

A dark surface at high illuminance can have lower luminance than a light surface at modest illuminance, which is why raising lamp output is often the wrong fix for a task that is hard to see.

The inverse square and cosine laws

For a point source, illuminance at a surface perpendicular to the beam is:

$E = \frac{I}{d^2}$

and when the surface is tilted by angle θ from the perpendicular, the cosine law applies:

$E = \frac{I \cos\theta}{d^2}$

Worked example. A 2,000 cd luminaire is mounted 4.0 m directly above a workbench. Illuminance at the bench directly below is E = 2,000 / 4.0² = 125 lux (about 11.6 fc). At a point 3.0 m horizontally from the centre, the distance is √(4² + 3²) = 5.0 m and cos θ = 4/5 = 0.8, giving E = 2,000 × 0.8 / 25 = 64 lux — roughly half, at only 3 m off-centre. This steep falloff is why luminaire spacing, not lamp wattage, governs uniformity.

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2. Regulatory Minimums

OSHA's general industry standard sets illumination requirements only in specific contexts, but the construction standard 29 CFR 1926.56 sets explicit minimums that are directly testable:

These are floors for safe movement and basic work, not design targets. The Illuminating Engineering Society (IES) recommended practice provides task-based design values that are much higher — typical office work in the range of 30 to 50 fc (300 to 500 lux), and fine assembly, inspection, or detailed drafting work substantially higher still. Design values increase with task difficulty, with reduced contrast, and with worker age, since the ageing eye transmits less light and scatters more.

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3. Contrast, Luminance Ratios, and Visual Performance

Visual performance depends on four factors: task size, contrast, luminance, and exposure time. Increasing illuminance helps only until contrast becomes the limiting factor — after which adding light adds glare without adding performance.

Recommended maximum luminance ratios:

Exceeding these forces continual adaptation as the eye moves, which is experienced as fatigue.

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4. Glare

The controlling insight for reflected glare is that it is a geometry problem, not a brightness problem. If the luminaire, the task surface, and the eye form the specular angle, no amount of dimming or brightening fixes it — the geometry must change.

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5. Colour and Flicker

Correlated colour temperature (CCT), in kelvin, describes the appearance of the light: lower values (2,700–3,000 K) look warm, higher values (4,000–6,500 K) look cool or daylight-like.

Colour rendering index (CRI), on a scale to 100, describes how faithfully surface colours appear under the source. High CRI is a safety requirement, not an aesthetic preference, wherever colour coding carries information — electrical wiring, gas cylinders, pipe marking, chemical labels, and product inspection. A low-CRI source such as low-pressure sodium renders colour so poorly that colour-coded systems become unreadable.

Flicker and the stroboscopic effect. Discharge lamps operating on alternating current modulate at twice the supply frequency. Where the modulation frequency is a multiple of the rotational speed of machinery, a rotating part can appear stationary or slowly rotating — a genuine machine-guarding hazard around lathes, saws, fans, and drive shafts. Controls include high-frequency electronic ballasts, distributing luminaires across different phases of a three-phase supply, and supplementing with incandescent or LED sources with low flicker. Perceptible flicker is also associated with headache, eyestrain, and, at some frequencies, photosensitive seizure risk.

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6. Measurement

Illuminance is measured with a photometer that is both cosine corrected (so light arriving at oblique angles is weighted correctly) and colour corrected (so its spectral response matches the photopic response of the human eye). An uncorrected sensor can be badly wrong under sources with unusual spectra.

Survey practice:

1. Allow discharge lamps to stabilise — typically 15 to 20 minutes.
2. Measure at the task plane and in the task orientation, not at the floor and not horizontally when the task is vertical.
3. Take a grid of readings to characterise uniformity, not a single reading; the average conceals dark spots that determine where work is actually difficult.
4. Shield the sensor from the operator's own shadow and body reflectance.
5. Record lamp type, age, and cleanliness. Lumen depreciation and dirt accumulation can reduce delivered illuminance by 30% or more over a maintenance cycle, and a cleaning and relamping schedule is frequently a cheaper fix than new luminaires.