Question 3 of 4: Visual Environment — Diamond-Sorting Task (Part B, choose 1 of 2)

Question 3: Visual Environment — Diamond-Sorting Task (Part B, choose 1 of 2) (20 marks: a–10, b–10)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a).

Given. Task luminance $L_{task}=400$ cd/m²; workbench (adjacent surroundings) reflectance $\rho=40\%$; workbench illuminance $E=500$ lx.

Find. The task-to-background luminance ratio, and whether it is appropriate for a fine visual-inspection task.

Approach. Convert the workbench's illuminance and reflectance to a luminance using the diffuse-reflecting-surface relation $L=E\rho/\pi$, form the ratio $L_{task}:L_{bench}$, and compare it against the recommended task-to-immediate-surroundings ceiling.

1. Background luminance from illuminance and reflectance. $$L_{bench}=\frac{E\times\rho}{\pi}=\frac{500\times0.40}{\pi}=\boxed{63.7\ \text{cd/m}^2}$$
2. Luminance ratio. $$\text{Ratio}=\frac{L_{task}}{L_{bench}}=\frac{400}{63.7}=\boxed{6.3:1}$$
3. Compare against the recommended ceiling. Recommended practice for a task and its immediate surroundings (the workbench the diamonds sit on) is a maximum ratio of about $3:1$ (up to roughly $10:1$ is tolerated to more remote background surfaces). The computed $6.3:1$ exceeds the $3:1$ immediate-surroundings ceiling.

The ratio is therefore not appropriate — the task area is more than twice as bright, relative to its immediate surroundings, as recommended. At this ratio the eye must continually re-adapt as it moves between the brilliant, high-luminance diamonds/task point and the comparatively dim workbench, which increases visual fatigue and slows the fine colour/size discrimination this task requires.

Part (b) — Visual comfort requirements and recommendations. Visual comfort for a fine, high-precision colour/size discrimination task under tweezer manipulation requires: (1) adequate task illuminance concentrated where the diamonds are actually viewed, using directional or adjustable task lighting rather than relying on general room lighting alone; (2) a luminance ratio between the task and its immediate surroundings kept within about $3:1$ (and no more than roughly $10:1$ to the general room background) so the eye is not forced to repeatedly re-adapt; (3) glare control — diamonds and glass dishes are highly reflective/refractive, so light sources must be positioned and diffused (not bare point sources) to avoid direct or veiling reflections off the dishes and tweezers into the worker's line of sight; (4) good colour rendering (high CRI) since the task is explicitly a colour-sorting task, so a light source with poor colour rendering would directly degrade task performance; and (5) minimizing flicker, since sustained close visual work under a flickering source accelerates eye fatigue. Recommendation: raise the workbench's own reflectance/brightness (a lighter-coloured work surface, or supplementary indirect light on the bench itself) to bring the $6.3:1$ ratio down toward $3:1$, while adding a diffused, high-CRI, glare-controlled task lamp aimed at the sorting point rather than increasing task luminance further (which would only worsen the ratio); the two changes together narrow the adaptation gap and directly address the eye-fatigue risk identified in part (a).