Question

3. There are two different formulas which can be used to calculate illuminance. Write both of the formulas. Explain the formula elements (include units). Explain when to use the one vs. the other when calculating illuminance levels in a room.

Answer 1

The Illuminance L can be calculated by two formulae,

1. By using Luminous flux

Illuminance, E = φ/A (unit is lux or lumen per metre square)

φ is luminous flux (unit is lumen)

A is Area (unit is metre square)

This defines Illuminance as luminous flux per unit area.

2. By using luminance

Illuminance, E = L.π/R

L is the luminance (Unit is Candela per metre square)

R is the reflectance factor of the surface.

Illuminance is the product of Luminance (L) and π (rad) divided by reflectance factor of the surface (R) .

The difference between the two methods can be understood as follows:

In 1st formula, as we know, luminous flux is a property of source, i.e light source in a room. Hence, this is important formula for checking if a certain light is appropriate for a perticular room or not.

But, the 2nd formula, tells us about luminance, property of surface to emit or reflect the light, this will vary from room to room, with placement of light, with the kind of surface the walls have.

Hence, when one is concerned about lighting a room both the formulae matters, luminance will give the required illuminance, this Illuminance will be used to calculate the luminous flux and hence we can select appropriate lights for the room.