Color rendering index, measured in Ra
The color rendering index reflects the relationship between color stimulus and color impression. The fascinating aspect is that lamps of different lighting technologies and the same light color (Kelvin) can produce different color impressions. Thus the colors of an illuminated object can have appear differently depending on the light source. The color rendering index describes the quality with which the light sources reproduce the individual colors.
To determine the Ra value, light sources are compared with a reference light source and classified using a reference color table. Lamps with an Ra value of 100 reproduce the reference colors the best. The index of sunlight or natural daylight is 100. This value is used as a comparison value for alternative light sources. Fluorescent lamps and LEDs often have lower values. This is important for purchasing lamps insofar as it’s possible to estimate with a given Ra value how the colors of the own furnishings are reproduced at home.
The table lists the average values of color temperature and color rendering index of different lighting technologies. This makes it clear that the same light color / color temperature (K-value) says nothing about the color rendering index / quality or the brightness. For the lamp purchase one can remember: an Ra value between 80 – 100 repoduces the colors of the illuminated objects the most natural.
The Ra scale reaches a maximum of 100:
In private households and the industry usually an Ra value of> 80 Ra is used. On the other hand the art and museum sector is more demanding. In order to ensure that pictures and motifs are not incorrectly reproduced, a very high light quality must be taken into account> 90 Ra. The maximum possible Ra value is 100 which among other things the former incandescent lamps achieved. But also halogen lamps and very high-quality LED light sources achieve predominantly good color reproduction values.
The color rendering index is less relevant for lamp purchases. It often doesn’t even appear on the packaging of the lamps. Only in the case of fluorescent tubes a reference to the color reproduction quality can be found. The internationally valid light color number for example “827”.
The most common are:
The first digit indicates the color reproduction stage which is determined by the color reproduction index Ra. The second and third number stand for the color temperature in Kelvin. Thus 827 means for example a color rendering of at least 80 Ra and a light color of 2700 K, so extra warm white.
Light intensity per unit solid angle, measured in candelas (cd)
The emitted light intensity of a light source (reflector) which can be perceived by the human eye is measured with candela. A candela (cd) is thus the unit of measure for the light intensity (luminous flux (lm) per solid angle). A light source has a candela when the illuminance is one lux (lx) at a distance of one meter. So you can conclude: the higher the candela value, the brighter the emitted light.
Candela (cd) is divided into 1,000 millicandela (mcd). Candela specifications on a lamp package are usually always specified in conjunction with a beam angle. This beam angle (in °) indicates the angle at which the lamp emits the light and which served as the basis for the evaluation. Since light bulbs, drop and globe bulbs are all-radiant, it is difficult to determine a candela value which is why it’s typically not given.
However with reflectors we’re talking about directed light and the calculation of the light intensity is easier to determine. The smaller the beam angle of a reflector, the greater the bundling of the light and thus the candela value. Typical Candela values (determined usually at a distance of 1m) are between 3 and 12 candelas in apartments, between 10 and 20 candelas in offices and mostly between 60 and 300 candelas in object and shop window lighting.