Lighting design often gets reduced to illuminance.
If the desk has enough lux, the corridor meets the target, or the fixture schedule looks reasonable, the lighting is sometimes treated as acceptable.
But illuminance is not the same as visual comfort.
A space can have enough light for the task and still feel uncomfortable because of glare.
That is where glare screening becomes useful. It does not ask only:
“Is there enough light?”
It asks a better question:
“Is the bright source likely to create visual discomfort for the observer?”
For offices, classrooms, workshops, control rooms, retail spaces, and daylight-heavy interiors, that question matters.
Glare is not just brightness
A common mistake is thinking that glare is caused only by a bright source.
Brightness matters, but it is not the whole problem.
Discomfort glare depends on several variables at the same time:
Source luminance
Background luminance
Apparent source size
Source position in the field of view
Observer direction
Contrast between the source and the surrounding field
A bright light in a bright background may be less disturbing than the same light in a dark background.
A small bright source directly in the line of sight may feel worse than a larger source outside the main viewing direction.
That is why a glare index calculation includes more than just source luminance.
The simplified glare index formula
The calculator uses a simplified single-source BGI / UGR-style formula:
Glare Index = 8 × log₁₀(0.25 / Lb × (Ls² × ω / p²))
Where:
Ls = source luminance, cd/m²
Lb = background luminance, cd/m²
ω = solid angle of the source, sr
p = position index
The formula shows three important relationships.
First, source luminance is squared.
That means glare sensitivity increases very quickly as the source gets brighter.
Second, background luminance is in the denominator.
A darker background makes the same bright source more uncomfortable.
Third, position index is squared in the denominator.
A source in a more visually sensitive position can have a much stronger effect than a source located away from the main viewing direction.
The supporting contrast check
The calculator also uses a simple luminance ratio check:
Luminance Ratio = Source Luminance / Background Luminance
When this ratio is high, the condition may have a contrast-related glare concern.
This is a practical warning because many real glare complaints come from contrast, not just absolute brightness.
For example, a bright LED strip against a dark ceiling, a window behind a monitor, or a high-luminance fixture in a dim corridor can all feel uncomfortable even if the average illuminance looks acceptable.
Example: workstation glare check
Suppose a workstation has a bright visible source in the user’s field of view.
Inputs:
Source luminance, Ls = 5,000 cd/m²
Background luminance, Lb = 200 cd/m²
Solid angle, ω = 0.01 sr
Position index, p = 1.5
Step 1: Square the source luminance.
Ls² = 5,000²
Ls² = 25,000,000
Step 2: Multiply by solid angle.
Ls² × ω = 25,000,000 × 0.01
Ls² × ω = 250,000
Step 3: Square the position index.
p² = 1.5²
p² = 2.25
Step 4: Divide by position index squared.
Ls² × ω / p² = 250,000 / 2.25
Ls² × ω / p² = 111,111.11
Step 5: Apply the background luminance term.
0.25 / Lb = 0.25 / 200
0.25 / Lb = 0.00125
Step 6: Calculate the logarithm argument.
0.00125 × 111,111.11 = 138.89
Step 7: Apply the glare index formula.
Glare Index = 8 × log₁₀(138.89)
Glare Index = 8 × 2.1427
Glare Index = 17.14
So the result is:
Glare Index ≈ 17.1
That falls into a moderate glare range.
The lighting may still be usable, but it is no longer clearly comfortable. People may notice the glare during longer visual tasks, especially if they are working on screens or looking in a fixed direction for long periods.
The contrast check tells the same story
Now calculate the luminance ratio:
Luminance Ratio = 5,000 / 200
Luminance Ratio = 25
A ratio of 25 means the source is much brighter than the background.
That does not automatically prove the design fails, but it is a strong warning sign. The source-background contrast is high enough that the engineer or lighting designer should review shielding, fixture placement, viewing direction, or background brightness.
What happens if the background is darker?
Now keep the same source luminance, source size, and position index, but reduce the background luminance:
Ls = 5,000 cd/m²
Lb = 100 cd/m²
ω = 0.01 sr
p = 1.5
Only one thing changed: the background became darker.
The background term becomes:
0.25 / 100 = 0.0025
Then:
0.0025 × 111,111.11 = 277.78
log₁₀(277.78) = 2.4437
Glare Index = 8 × 2.4437
Glare Index = 19.55
The glare index increases from about 17.1 to about 19.6.
Nothing happened to the light source itself. It did not become brighter. It did not become larger. It did not move.
The space simply became darker around it.
That is the practical lesson:
Glare is strongly affected by contrast.
Common engineering mistake
The most common mistake is checking only illuminance and ignoring luminance.
Illuminance tells you how much light lands on a surface.
Luminance tells you how bright a surface or source appears to the eye.
Those are different design questions.
A desk can receive enough lux while the user still sees a bright luminaire, window, or reflection that causes discomfort.
Another mistake is treating one glare number as a full design approval.
A simplified glare index is useful for screening, comparison, and early design review. But real visual comfort still depends on the actual scene, observer location, task direction, fixture optics, daylight conditions, surface reflectance, and layout.
The third mistake is ignoring source position.
A bright source directly in the user’s field of view is not the same as a bright source outside the main viewing direction. Position matters because discomfort glare is tied to how the eye sees the source, not only how bright the source is.
Practical design responses
If the glare index is moderate or high, the solution is not always “reduce light output.”
Better options may include:
Use better shielding or louvers
Move the fixture out of the main viewing direction
Increase background luminance to reduce contrast
Change fixture optics
Add shading for daylight glare
Reorient workstations
Reduce direct view of high-luminance sources
Use indirect or diffused lighting
Review screen reflections
The right fix depends on the cause.
If the problem is source brightness, reduce or diffuse the source.
If the problem is contrast, improve the surrounding brightness balance.
If the problem is position, change fixture placement, observer direction, or shielding.
Practical engineering takeaway
Glare screening is not a replacement for full lighting simulation, but it is a useful early warning tool.
Before accepting a lighting layout, ask:
- Is the bright source directly visible?
- Is the background much darker than the source?
- Is the source large enough to matter visually?
- Is the source located in a sensitive viewing direction?
- Are people working on screens or long-duration visual tasks?
- Could shielding, layout, or surface brightness reduce the problem?
If the answer to several of these is yes, checking only lux is not enough.
For a quick first-pass review, you can use the Glare Index Calculator
It calculates a simplified glare index from source luminance, background luminance, solid angle, and position index, then classifies the result so you can quickly judge whether the condition is likely to be very low, low, moderate, high, or very high glare.
