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Scotopic/Photopic ratios: why do we have them and when are they used?

July 2019, by Lee Walker, Applications Engineer

“Does having a higher SP ratio make your lighting design better?” 


Barely a week goes by without a new controversial article or contradictory research paper covering the virtues - or pitfalls - of LED lighting, specifically the spectral power distribution (SPD) and blue light content of white LEDs. Within this hot topic, Scotopic and Photopic ratios usually make an appearance.

Scotopic/Photopic ratios, more commonly known as S/P ratios, are a fairly new addition to lighting standards, BS 5489-1 - introduced in 2013 and are used to understand the level to which a white light scheme can be lit, relative to SON or SOX.

So, what are they intended for and why they are specified on some lighting schemes and not on others?  S/P ratios should provide a levelling of the LED playing field, but do they? 

Understandably, responses to the questions above are mixed. 

The majority believing that the cooler the colour temperature, the brighter the light appears and as a result, the fewer lumens needed to meet the lighting requirements. But, is this now a controversial view given health concerns associated with colour temperature and especially, blue light?  

S/P Ratio explained

Let’s first tackle the ‘S’. Scotopic. Using your rods, scotopic vision is the term used for low light level vision, where the eye utilises only the rod receptors for sight, picking out movement but no discernible colours or definition. 

The ‘P’. Photopic. Using your cones, photopic vision is the vision of your eye under well-lit conditions and is responsible for colour perception. 

When artificial street lighting is used, we create lighting levels which lie somewhere between photopic and scotopic vision, this transition is known as mesopic vision. During this state, both photopic and scotopic responses are required to enable you to see well.

Mesopic = Photopic + Scotopic

How do we calculate S/P Ratios?

In practice, we have no logical method for measuring mesopic lumens. Imagine trying to measure a moving target – any value would only be relevant to a specific quantity of light and, as the light varies, the eyes response curve will too. Until we meet photopic or scotopic vision. Therefore, the only fixed points we can effectively determine are the eyes response curves for photopic and scotopic conditions.


Scotopic lamp lumens


Photopic lamp lumens


The Lighting Industry Association states, “A method to indicate how good a light source will be under photopic, mesopic and scotopic conditions is the S/P ratio, which is the scotopic lamp lumens divided by the photopic lamp lumens. If this ratio is equal to 1 the lamp performs equally under photopic, mesopic and scotopic conditions, and the reading on your illuminance meter will also be correct for all of these conditions. A value greater than 1 indicates that the lamp produces more scotopic lumens than photopic lumens, and your meter will underestimate the scotopic illuminance levels. Conversely, a value less than 1 indicates that the lamp produces more photopic lumens than scotopic lumens and your meter will overestimate the scotopic illuminance levels.”

Some representative S/P ratios are given in the table below. Note these are generic and in practice, the values for the specific light source should be used:

  Light source S/P ratio
  Incandescent 1.36
  Fluorescent (3500K) 1.36
  Fluorescent (5000K) 1.97
  Metal Halide (warm white) 1.20
  Metal Halide (daylight) 2.40
  High Pressure Sodium 0.65
  Low Pressure Sodium 0.25
  LED (3500K) 1.39
  LED (6000K) 2.18

Reference[1]: Technical Statement LIA TS 24 Issue 1 - 05/2013 


Why use S/P ratios?

The use of S/P ratios, the brightness of light the brain perceives for different colours of light, enables us to lower energy consumption and still see well.

To understand further, we need to take a whistle-stop tour through the history of light because, believe it or not, there was a time when we relied on something other than LED to light for us! 

Content banner image History of light 2340x800px

The 15th century: and as far back as 1417, lanterns with candles were used on the streets of London on winter nights, with a colour temperature of around 1800K. 

The 18th century: fast forward to gas and oil lighting, with a CCT of around 2800K. 

The 19th century: and electric street lighting arrived, from warm sodium (1700-2000K CCT), to whiter lamps (2700K+ CCT) and finally, LED, with a far broader range, but typically sitting between 3000-5000K CCT. However when LED first arrived it was common to have colour temperatures of in excess of 5000k+. This was the only way to get enough light out of the fledgling technology, but did result in some less than perfect lighting installations, with many users complaining of the harsh cold new luminaires.

This does mean, for the majority of time, exterior lighting has been warm! 

White light

As lamp and LED technology has improved we are able to produce white light with greater and greater efficiency and so the need for cold CCTs dissipated. 

Our research and understanding of the physical and psychological effects of artificial light on the human body and the environment around us also developed. This has led to greater consideration of how we interact with the light spectrum, for example how the eye works in different light conditions and what we need to be able to see in low-level lighting environments - including to walk and drive. We are also now aware that light has an impact on how we feel - for example safe and secure or alert and awake. And more recently, we have investigated the impact on sleep cycles (our circadian rhythm) and health. These technological developments have come with significant improvements in energy, capital and maintenance costs and, of course, in the quality of light being offered.

White light and street lighting 

The use of white light for street lighting has increased as the efficacies and colour properties of LED’s have improved, providing comparable light output to a traditional high-pressure sodium scheme – for a reduced amount of energy consumption. With the advent of white lamp sources replacing sodium, the British Standard for lighting, BS5489 2003, acknowledged that whiter light created a brighter looking environment and so, suggested that lighting designers, using a white light source, CDO-TT or CosmoPolis for example, on a pedestrian focused lighting scheme (S, or P classes as they are known now), drop a whole lighting class. “Interesting” you say.

Why only P class roads?

Our two classifications of roads in the British Standard and European Normative are M and P. 

M class roads as generally used by vehicles at speed (motorways, trunk roads, A roads etc.) These are calculated using a luminance method in cd/m square. Luminance measures the light reflected from the road surface and objects are seen in silhouette against this bright background. Colours and edges are less important in this context as you need the contrast.

On the other hand, P class roads are defined as mixed pedestrian and vehicular traffic (residential roads), calculated using an illuminance method measured in luxIlluminance is considered as the amount of light “falling” onto an object and its surroundings. Here is where a higher S/P ratio can make a difference. Colours become more distinguishable, outlines more vivid and brighter.

Is this the way towards energy savings?

Energy saved is excellent news BUT, sometimes dropping a lighting class and halving your light levels without considering other implications, is not always the most appropriate thing to do.

As the use of LED became more prevalent, and with a deeper understanding of human interaction with certain light spectrums, this “drop a class” method was replaced in the current 2013 version of the British Standard with S/P ratios.

Street lighting energy bills can be expensive and if you make your purchase decision based only on photopic lumens, you may end up overlighting, consuming much more power than is necessary. A lower lumen LED luminaire could save you money, operate more efficiently, while providing exactly the right amount of light, well this is the theory.

Now we are getting the benefits of the S/P ratio

Having covered the background and where it all started, in short, based on the spectral output of the light source, a lamp or LED is assigned an S/P ratio. This is partly why you don’t need as many LED lumens to compete with lamp lumens; the quality of the light dictates how we interpret the brightness of the lit scheme, letting us lower the lighting levels needed to achieve the same effect. This in turn allows you to adjust the levels for that class and deliver major energy and capital cost savings! So, good news on all fronts! Isn’t it?

S/P Ratio and Illuminance when Ra ≥60
 P   class   Eav Photopic illuminance (lux) for Ra ≥60 according to S/P ratio of lamp
0.2 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
  P1 15 15 14.3 14 13.7 13.4 13.1 12.8 12.6 12.3 12.1 11.8 11.6 11.4
  P2 10 10 9.4 9.1 8.9 8.6 8.4 8.1 7.9 7.7 7.5 7.3 7.2 7
  P3 7.5 7.5 7 6.7 6.5 6.3 6 5.9 5.7 5.5 5.3 5.2 5 4.9
  P4 5 5 4.5 4.3 4.1 4 3.8 3.7 3.5 3.4 3.3 3.2 3.1 3
  P5 3 3 2.6 2.5 2.3 2.2 2.1 2 1.9 1.8 1.7 1.7 1.6 1.6
  P6 2 2 1.7 1.6 1.5 1.4 1.3 1.2 1.2 1.1 1 1 0.9 0.9

Reference[2]: PLG03 Lighting for Subsidiary Roads: Using White Light Sources to Balance Energy Efficiency and Visual Amenity, ILP publication


Applying the theory

Content banner image Photopic and scotopic curves 2340x800px

Reference [3]: Technical Statement LIA TS 24 Issue 1 - 05/2013 

Here [3] we can see the eyes response is heavily peaked in the blue/green part of the spectrum.

The closer a lamp sources’ spectral output matches this photopic response, generally the higher its S/P ratio and thus the more you can reduce your lighting levels and the more money and energy you can save. The argument is won, everybody starts specifying lamps and LEDs with an S/P ratio greater than 2.

Let’s look at an example of how S/P ratios can help save energy

BS EN 5489:2013 allows for the use of S/P ratios, whereas EN13201, the document that the BS is based upon does not. If we compare a typical lighting scheme, firstly designed in accordance with EN13201 to class P4.

Content banner image P classes example table 2340x800px
Content banner image P classes example diagram 1 2340x800px

Reference [4]: BS EN 5489:2013

We haven’t applied an S/P ratio to this scheme and the total wattage is 112W, now compare this to a scheme designed to BS5489:2013, which does allow for the use of S/P ratios. We have used the same 4000K LED product and applied its S/P ratio of 1.5.

Content banner image P classes example diagram 2 2340x800px

Reference [4]: BS EN 5489:2013

The energy consumed by this scheme has reduced to 80W, a reduction of 32W, resulting in approximately 30% less energy, for the same lighting levels, as allowed by the standard and the same number of light fittings, an easy argument for energy savings and the associated cost reductions.

With this example we have demonstrated, that there is no real right or wrong approach, but there is a way to save energy and provide quality lighting in accordance with the British Standards instead of using the EN13201 standard.

When is the right time to use an S/P ratio?

Wait! Is saving money our only concern? Remember S/P ratios should only be applied to P lighting classes. These are the classes designed for people, used by people and lived in by people.

By default, lamps or LEDs with high S/P ratios will have a high proportion of their spectral output in the blue/green area; generally, as your colour temperature gets cooler, the S/P ratio improves. 6000K would have a higher S/P ratio than 3000K for example, so shouldn’t we all be specify 6000K light sources?

In today’s playing field where we have a lot more research available on the effect of different spectral elements of light, this wouldn’t sit well. We are now increasingly aware of the potential issues caused by blue rich LED sources.

This should bring us to question, what should we be mindful of when deciding to specify high S/P ratio light sources? In effect, what are the pros and cons that we must assess going forward, here’s our list, can you think of anymore?

Vision: what does applying the S/P ratio really mean for our sight in practice. Using high blue content LEDs does help to improve peripheral vision which is obviously important for hazard perception, however the subsequent lowering of lighting levels when using high S/P ratio light sources actually decreases our focal vision, the part responsible for sharp, in focus images, making it harder to see details as well. Conclusion: It’s not always safe to lower the lighting levels too much.

Standards: It is important to understand that unless specifically stated within a standard or design specification, you should not adjust the lighting requirements based upon the S/P ratio.

Preference: research has shown a preference for more humancentric, warmer white light sources in people orientated spaces. 3000K and lower, which is at odds with the high S/P ratio light sources.

Blue light: we have touched on it but there is a glaring issue concerning blue light content. High blue content LEDs can be responsible for more sky glow, short wavelength light sources “scatter” more in the atmosphere, meaning more light pollution.

Age: related to the point above consider older and younger users. Research has shown that children have a higher sensitivity to blue light and although emissions may not be harmful, blue light (between 400nm and 500nm) may be dazzling and could induce photochemical retinopathy, which is a concern, especially for children below three years of age. The elderly population may experience discomfort with exposure to LED systems, as we age, our eyes ability to absorb blue light lessens, so high S/P ratios can make performing tasks, like driving, even more difficult for the elderly.

Use S/P ratios to help make informed decisions

A good point to return to my original question, Does having a higher SP ratio make your lighting design better?” 

Unfortunately,it’s not as simple as a yes or no: there is no definitive right or wrong. But we should be mindful when applying S/P ratios to consider the space and the user

It is still possible to reduce lighting levels AND light a space well and comfortably. Using good quality warm LED sources. 

It may be wise to steer clear of the 1.8 or 2+ ratios and find a middle ground that will satisfy the paying customer with energy savings and the people expected to use and live in the space, who unfortunately seem to be often forgotten.


[1&3]: Technical Statement LIA TS 24 Issue 1, May 2013

[2]: ILP PLG03 Lighting for Subsidiary Roads: Using White Light Sources to Balance Energy Efficiency and Visual Amenity, 2012

[4]: BS EN 5489:2013

[5]: Roadway LEDs Address Unintended Effects on Flora and Fauna, February 2018 / Impact of lighting on flora and fauna, January 2017

[6]: Human & Environmental Effects of Light Emitting Diode Community Lighting, American Medical Association, 2016 / Human Responses to Lighting Based on LED Lighting Solutions, Public Health England, SLL, CIBSE, April 2016 / Preliminary Opinion on Potential Risks to Human Health of Light Emitting Diodes, European Commission, The Scientific Committee on Health, Environment and Emerging Risks, July 2017