How to Choose the Color Temperature of a Solar Street Light

Color temperature is the parameter that starts the most arguments when specifying a solar street light. Some buyers insist that whiter means brighter; others are certain that only yellow light cuts through haze; many simply copy whatever figure appears on the drawing. In practice, color temperature is a question of optics, human perception and local codes at once - and on a solar fixture it ends up on the PV panel and battery bill as well. This article works from first principles through to how to fix a range for a specific site.

 

What Is the Color Temperature of a Solar Street Light?

From the spectral energy distribution and the color of a light source, we can introduce the quantity known as color temperature, which describes the color of a source. When the color of the light emitted by a source is the same as the color radiated by a blackbody at a given temperature, that blackbody temperature is called the color temperature T of the source - color temperature for short - expressed on an absolute temperature scale. For certain sources (mainly gas-discharge lamps with strong line spectra), the color of the light emitted is not exactly the same as that of blackbody radiation at any temperature (their chromaticity coordinates differ), so the general concept of color temperature cannot be used to describe their color. For ease of comparison, however, the concept of correlated color temperature was introduced.

 

If the light emitted by a source comes closest in color to blackbody radiation at a given temperature - that is, the chromaticity distance on the uniform chromaticity diagram is the smallest - then that blackbody temperature is called the correlated color temperature (CCT) of the source. Correlated color temperature is a relatively coarse way of describing color, but it does express color to a certain extent.

 

The 3000K, 4000K and 5800K ratings marked on solar street light products refer precisely to the correlated color temperature (CCT). CCT describes the color of the light, not the brightness of the luminaire - two 3000K fixtures can differ twofold in brightness. This is the point most often confused during product selection.

 

 

Solar Street Light Color Temperature: What Effect Does Each Range Have?

Cool white light contains a larger proportion of blue, and warm white light a larger proportion of yellow. Two main differences follow from this: penetrating power and how the eye perceives the light.

 

Among the common options, 2700–3000K is yellowish and looks much like a conventional sodium lamp. The light is soft and has little impact on nearby residents or on the night sky, so it is widely used for courtyards, residential roads and locations that see fog or rain. At 3500–4200K the light color approaches moonlight, striking a balance between clarity and comfort, which makes it the most common choice on municipal roads. Above 5000K the light appears white or even bluish; it looks the brightest and offers good long-distance recognition, but it is the most contentious option in rain and fog, and in areas immediately adjacent to homes.

 

The human eye perceives color temperature differently under different levels of illuminance. Indoors, at a given illuminance, an excessively high color temperature feels dreary, cold and gloomy, while an excessively low one feels stuffy, dim and oppressive. Outdoors, this perception needs some correction: the comfort zone shifts slightly toward higher color temperatures.

Another change is under way. LED technology has continued to advance rapidly in recent years, chip efficacy keeps improving, and energy savings are assured. The conflict between efficacy and color temperature is no longer the main issue in application; whether the color temperature suits the environment has become the focus of attention.

 

 

How Does Color Temperature Affect the Human Eye and Safety?

Let us start with penetrating power. Rayleigh scattering explains why the sky is blue: the intensity of Rayleigh scattering is inversely proportional to the fourth power of wavelength, so in the solar spectrum the shorter-wavelength blue-violet light scatters more strongly than the longer-wavelength red light, and among the shorter wavelengths blue carries the most energy. When the sky clears after rain or on a crisp autumn day (when there are few coarse particles in the air and molecular scattering dominates), air molecules scatter light strongly by Rayleigh scattering, blue light is diffused throughout the sky, and the sky appears blue.

 

Put simply, the shorter the wavelength, the more readily light scatters; the longer the wavelength, the more readily it penetrates. PM2.5, the main contributor to haze, consists of many fine particles that likewise cause light to undergo Rayleigh scattering. Cool white light has a high proportion of blue and warm white light a high proportion of yellow, so in theory the lower the color temperature, the stronger the penetrating power - and in haze, high-color-temperature LED street lights penetrate somewhat less well.

 

 

Now for glare and lumen depreciation. It is commonly assumed that LED street lights penetrate fog poorly and that they cannot handle crosswalk markings, are more glaring and suffer heavy lumen depreciation. In practice this is not necessarily so. Looking at the lighting performance of 4000K LED street lights, and at a comparison between 3000K LED street lights and high-pressure sodium lamps, we find that as long as the light distribution is properly controlled, an LED street light seen in peripheral vision from the direction of travel is neither glaring nor does it produce uncontrolled glare, and illuminance across the road surface is uniform.

 

Lumen depreciation also has to be distinguished from its imitation. When looking at depreciation, it is important to tell true depreciation from false depreciation. Data from a number of LED street light test projects show that without cleaning the fixtures, false depreciation is considerable, while the depreciation of a fixture aged indoors is smaller than the "depreciation" caused by soiled lenses. In other words, a road surface that looks dimmer is not necessarily suffering chip degradation - in many cases a layer of dust has simply accumulated on the lens.

 

 

What Standards and Application Requirements Apply?

Taking technical conditions, climatic conditions, current environmental conditions and human perception together, experts suggest that the color temperature of LED street lights should be selected within the range of 2800–4200K. This color temperature does not have much effect on efficacy, guarantees penetration through fog and haze, and suits the visual perception of users in China.

 

Formal projects are also subject to standards and tender documents. Road lighting design is based on the relevant national and industry standards, and some local dedicated lighting plans directly stipulate color temperature ranges; the actual selection should be governed by the code provisions in force where the project is located and by the tender documents.

The results of actual testing by experts, however, are somewhat surprising. In theory yellow light has higher transmittance than white light in pure air, but testing and analysis show that, as far as transmittance in the context of practical road lighting is concerned, the conclusion runs counter to long-standing assumptions:

 

• There is no difference in transmittance between yellow light (for example, sodium lamps) and white light (for example, high-color-temperature white LED lamps) in clear weather;
• At least for fog and cloud of a certain density, and in rain and snow, there is likewise no difference in transmittance between yellow and white light;
• Because of visual error or the sheer variety of weather conditions, occasional exceptions that are hard to explain may arise as clear weather evolves toward extreme fog, cloud, rain or snow.

 

In other words, the claim that "yellow light penetrates haze better" does not hold up well once it is placed in the context of practical lighting across a whole road. Of course, transmittance is only one factor. Whether white or yellow light is better for road lighting is still difficult to conclude, because too many factors are involved and they are too complex - there are all kinds of objective influences as well as subjective human factors, especially the differences among large numbers of individuals. There can be no result on which everyone agrees; opinions will always differ.

 

 

How Does Color Temperature Affect the Cost and Autonomy of a Solar System?

A solar street light differs from a mains-powered one: the color temperature bill ultimately lands on the PV panel and the battery. The chain runs like this - color temperature affects the luminous efficacy of the LED (lm/W); efficacy determines how much power is needed to reach a given illuminance; power determines the fixture's daily energy consumption; consumption in turn determines the PV panel rating and battery capacity; and these finally determine overall cost, volume and weight, and how many consecutive overcast and rainy days the light can keep running. In a solar street light, then, color temperature is not merely a question of whether the light looks good - it is a system configuration issue.

 

Fortunately, LED technology has advanced rapidly in recent years, chip efficacy keeps improving and energy savings are assured, so the conflict between efficacy and color temperature is no longer the main issue in application. Whether the color temperature suits the environment in which the light is used has become the key point in selection.

 

It is also worth factoring lumen depreciation into the calculation in advance. The false depreciation discussed earlier shows that a real drop in illuminance usually comes from dirt accumulating on the lens. If a system is configured on a long-term maintenance-free basis, margin has to be left in the panel and the battery; otherwise illuminance on the road will fall off after a few years, which in effect shortens the design life.

 

One further reminder: whatever color temperature is chosen, proper light distribution control is a prerequisite. With suitable optics, an LED street light is neither glaring nor does it produce uncontrolled glare, and illuminance across the road surface is uniform; with poor optics, even the mildest color temperature will not help.

 

Conclusion

All things considered, after years of research and practical application, Yahua Lighting recommends a color temperature of around 5800K: it delivers high efficacy while remaining bright and comfortable, and drivers are better able to stay alert in that environment, reducing the number of accidents; conversely, warm light at an excessively low color temperature feels stuffy, dim and oppressive, and easily induces fatigue. For specific projects such as residential areas and fog-prone road sections, however, we still recommend adjusting to the situation within the general 2800–4200K range, and deferring to the local codes and tender documents.