The table below demonstrates the relationship between input power and luminous flux for these bulbs.

Different types of light bulbs emit different intensities of wavelengths. Spectrometers are used to measure the proportions of each colour of light emitted and results are presented as graphs showing the intensity of each colour. These graphs are known as light spectra.

The most polluting light sources in terms of sky glow are those that emit a lot of blue light. This is the case for white lights. White light is actually composed of all colours of the rainbow. Because the human eye perceives blue light less well than yellow light, a light source must contain a high proportion of blue light to appear white. It is the whiteness of light that permits a faithful colour rendering. Unfortunately, the bulbs that offer the best colour rendering are also those that pollute the most! This is why the use of these types of bulbs is sometimes prohibited on the IDSR territory.

Aim the light where it is needed most. Choose luminaires according to their photometric characteristics, i.e., how the luminaire controls the luminous flux produced by the lamp.

For roadways, select luminaires that emit a minimum of light on the house side of the road and a maximum along the street side (see the figure below). The photometric report supplied by the manufacturer describes the distribution of luminous flux around the luminaire. Use this report to choose luminaires that fit your requirements.

Finally, you can save money by choosing luminaires that are both easy to maintain and equipped with high-performance parts. Make sure luminaires are composed of durable materials. Ovoid polycarbonate and acrylic lenses and opt for tempered glass. Also, check that the sealing is tight and conforms to the IP65 specification.

Demystification of the IESNA classification system

The design of a luminaire determines how the light will be emitted into the environment. Since designs vary widely, the distribution pattern of luminous flux around the luminaire can also vary greatly.

The IESNA has proposed a classification of luminaires according to their “cutoff class”. This system primarily depends on the amount of light emitted in the potential glare zone (in the 10° angle below the horizon). There are four categories:

• non cutoff
• semi cutoff
• cutoff
• full cutoff

Since this classification can not be used to define the exact proportion of luminous flux emitted above the horizon, only the “full cutoff” class guarantees that absolutely no light output is directed to the sky. Hence this system is often used incorrectly to simply limit the choice of a luminaire to one that does not light up the sky. It is important to realize that a luminaire in another class could emit less than 1% of luminous flux towards the sky while offering improved downwards efficiency. That is why the regulations adopted in the region of Mt. Megantic are based on a limit of 1% light emitted above the horizon (not just on the IESNA class). These by-laws thus allow for more options in high-performance luminaires. Don't forget that the photometric report contains all the pertinent data!

The IESNA first proposed the above four classes in 1963. In the last decade, however, light pollution (particularly light trespass and glare) became a real problem in most communities. Complaints were skyrocketing and new descriptions were required to clarify luminaire characteristics.

The IESNA thus revised its categories in 2007 to allow a more precise evaluation of the distribution pattern of light output. A brief article on the new classification system appeared in the July 2007 issue of the magazine Électricité Québec.