Measured Light vs. Perceived Light

Understanding Light Perception

The human eye responds to low light levels by enlarging the pupil, allowing more light to enter the eye. This response results in a difference between measured and perceived light levels.

A lamp that is dimmed to 10% of its maximum measured light output is perceived as being dimmed to only 32%. Likewise, a lamp dimmed to 1% is perceived to be at 10%.

Design Example Scenarios

At full brightness, the measured light in a space is 60 foot-candles. At the lowest dimmed level, 10% perceived light is desired.

• 1% measured light (0.6 fcd) is perceived as 10% (desired result)
• 5% measured light (3 fcd) is perceived as 22% (2x brighter than desired)
• 10% measured light (6 fcd) is perceived as 32% (3x brighter than desired)

Visualizing Light Dimming

The relationship between measured and perceived light is illustrated in a graph. The Y-axis represents 'Measured Light' (from 1% to 100%), and the X-axis represents 'Perceived Light' (from 0% to 100%).

The graph displays several dimming curves, indicating how different dimming strategies affect perceived brightness:

• 1% Architectural dimming: This corresponds to very low measured light levels, where perceived brightness is significantly higher than measured.
• 5% High performance dimming: This range covers slightly higher measured light levels.
• 10% Lighting management dimming: This covers a broader range of measured light, offering a different perceived brightness curve.

A curved line shows how perceived light is non-linear with measured light, particularly at lower levels. For instance, achieving 10% perceived light requires only 1% measured light in architectural dimming, while higher measured light percentages are needed for the same perceived brightness in other dimming types.

An illustration depicts a light meter device displaying a reading of 0.875. The concept of perceived light is symbolized by an eye ?️.

Formula for Perceived Light

Perceived Light (%) = 100 x √ (Measured Light (%) / 100)

Source: IESNA Lighting Handbook, 9th Edition, (New York; IESNA, 2000), 27-4.