There are also other light sources, i.e. decorative bulbs or arc lamps in cinema projectors, as well as mercury lamps formerly used in warehouses and production halls, and on the railways.
It is important to note is that the entire system of socket and LED bulb will have a much lower efficiency than the LED diode, which is only a part of such a device. LED light sources of good quality have an efficiency of more than 80lm / W. In case of industrial solutions they should have efficiency of more than 85-90lm / W.
Color rendering index CRI – it determines how well a light source reproduces the colors, it is a number between 0 and 100. The abbreviation “CRI” means Color Rendering Index. The value of 0 refers to a single color light (monochrome), i.e. single electromagnetic wavelength of visible range (380 – 760 nm), the highest possible value of color rendering index – 100 – refers to white light with continuous spectrum.
Along with the increase in the value of the index, the color of items is better and more natural. Low values of the CRI index of a specific light source make the colors look like faded, and in exceptional cases they are seen as black. For example, if we illuminate a wall built of red brick with light blue only, then this wall will be very difficult to see, not to mention the inability to determine the color of the wall itself.
Low-pressure sodium lamps have the lowest color rendering index among the commonly known and used light sources, while the most common in street lighting high-pressure sodium lamps provide color rendering of 20-30. LED lamps used externally provide a CRI of 70 or more.
In case of office applications, we can find it for fluorescent lamps labeled with a 740 or 840 code, they provide a light with a CRI of 70+ and 80+. In such applications of a LED, the smallest value of color rendering index is 80+, there are also LEDs with color rendering of 90+, and even special modules with 95+ CRI.
Recommendations associated with a color rendering index:
Very high CRI – 90+ recommended for works, in which recognizing specific colors is important, e.g. shops, printing industry, control of color templates, art studios.
High CRI – between 80 – 90, recommended for office work in classrooms and lecture halls, as well as in the textile industry and in other works requiring precision. It is the most common recommendation, it concerns most of the works performed inside and places where people live for a long time.
Medium and low CRI – below 80, however higher or equal to 40 used for industrial works, in which distinguishing colors has no significant meaning.
The color temperature of light – determines the color of white light, however, does not include brightness, the proper unit is Kelvin [K].
The color temperature is closely related with the theory of Perfect Black Body, which is non-existent in reality, physical body that absorbs entire electromagnetic radiation directed on it. In the case of heating such a body above a certain temperature, expressed in Kelvins, it will generate electromagnetic radiation in the visible range, with a maximum, that increases along with an increasing temperature.
For example, the light source providing white light with a color temperature of 4000K shines same as the Ideally Black Body heated to such temperature. In case of LED, it does not mean however, that such a light source achieves so high temperature at any point of the structure.
For sources providing white color of light we can distinguish three groups:
Warm white color – According to the standard PN-EN 12464-1 regarding the lighting of indoor work places, it is the color temperature of below 3300 K. The LEDs with this color have a similar color temperature as the light provided by a classic light bulb. LED lamps can perfectly replace old, energy-consuming light sources.
Neutral white color – according to the standard PN-EN 12464-1 it is the color temperature with range from 3300 K to 5300 K. This is the color most commonly used for fluorescent tubes. In case of LED diodes color temperature range is usually narrower and is 3800-4200 or 4000-4500 K. The light of neutral color is closest to the pure white light, ideal for all types of stores and commercial premises.
Cold white color – the standard PN-EN 12464-1 defines it as the color temperature of over 5300 K. It is a cool-white light, but in case of light-emitting diodes, it provides more light than the warmer color, so it is especially suitable for outdoor applications and places, where the most important is to provide the greatest possible amount of light, with the lowest energy consumption.
In addition, the standard PN-EN 12464-1 (Light and lighting, lighting of work places Part 1: Indoor work places) contains requirements for color temperature depending on the value of the average illuminance. Along with the increase of average illuminance the color temperature of source should also increase.
Average illuminance on the working surface:
• Below 300 lx -> Color temperature should be lower than 3300 K – it is warm white color
• 300 ÷ 750 lx -> Color temperature within the scope 3300 ÷ 5300 K – recommended neutral white color
• Above 750 lx -> Color temperature should be higher than 5300 K – cold white color