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'What are S/P Ratios?'

SP Ratios – what is an SP Ratio?

First things first, SP stands for Scotopic/Photopic. If you’re new to the industry or are not that involved you may not know what Scotopic and Photopic is.

Let’s break these down:


Scotopic:


This relates to vision in a dim light, for example at night. The word scotopic originates from the greek word skotos – which means darkness. Your eyes use mainly your rods (more on these in a sec) to see in low light conditions.


Photopic:


This is basically the opposite of scotopic – vision in day light conditions or under bright lights and your eyes use the cones rather than the rods to see.

We have another Opic to consider and that is Mesopic. This is a combination of both Scotopic and Photopic vision in dark low light conditions – not daylight and not complete darkness. You could consider most highway lighting to fall within this type of vision.


How does the SP ratio affect lighting design?


Lamps / light sources are assigned an SP ratio - this ratio is a calculation of the Scotopic lumens the lamp produces divided by the Photopic lumens. If the ratio is 1 the light source performs just as good under Scotopic conditions as Photopic. The higher this number the better your eye performs under the light source.


Street Lighting Design:


Typical LED street lighting lanterns on the market with a colour temperature of 4000K have an SP ratio of 1.6.

When producing street lighting designs the levels required to meet the standards will vary depending upon the light source used and its SP ratio. Let's look at an example:

Typical residential street requires a lighting class of P5; the levels below are required to meet this lighting class:


Before SP Ratio







SP Ratio applied







Eav = Average lux required

Emin = Minimum lux required


With the 1.60 applied we can significantly save on energy, number of columns, lanterns and reduce the impact the installation has on the environment because we require less light.


Typical SP Ratios for LED Light Sources:

4000K Neutral White LED = 1.6

3000K Warm White LED = 1.3


We hope that has assisted with the term SP ratio and how it impacts the exterior lighting industry.


If you require an exterior lighting design - get in touch via peter.williams@wlclighting.co.uk or call us on 01234 630039 and we'll be happy to assist.


All the best,

Peter

Williams Lighting Consultants Ltd.

Company No. 10736462 

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