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'What are S/P Ratios?'

Updated: Nov 2, 2020

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 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 in these conditions.


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 (Updated 2020):


SP ratios were a required consideration within the BS 5489-1:2013 standards for road lighting. The recently revised BS 5489-1:2020 (check out the significant changes here) now considers all light sources to have an SP ratio greater than 1 and therefore you no longer need to adjust the lighting level requirements.... unless you use a light source with an SP ratio lower than 1.


To summarise this, SP ratios and lighting levels only need consideration when the light source you are proposing has an SP ratio lower than 1.


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 and we'll be happy to assist.


All the best,

Peter

Williams Lighting Consultants Ltd.

Company No. 10736462 

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