Efficacy

Question 1

Define efficacy

Answer 1

Efficacy is lumens per watt. The theoretical maximum for 683 lumens per watt which is only possible for 555 nanometers (green). Due to relative spectral sensitivity all other colours will be less. The best fluorescents are about about 90-100 lumens per watt.

Question 2

What is the efficacy for different lighting types.

Answer 2

Low pressure sodium 200 lumens per watt.
Led a little over 100 lumens per watt.
Fluorescent a little under 100 lumens per watt.

Question 3

What is utilisation factor

Answer 3

The proportion of the flux generated by the luminaire that reaches the working plane. This can often be 0.5.

Question 4

Explain the leni system of part L compliance

Answer 4

Instead of rating energy use by fitting efficacy then it is rated per watt per square meter per year. The space is rated for the minimum illuminance requirements for that type of space and if the designer chooses to over-illuminate they still have to calculate efficacy for the minimum luminance levels. The advantage of this is it deals more directly with the issue that matters watts used per year.