Home Gadgets Choice of LED bulbs. 3. Spectra of light sources

Choice of LED bulbs. 3. Spectra of light sources

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Ideally, you need a spectrophotometer to evaluate the quality of a lamp’s emission spectrum. As a last resort, you can use spectrophotometers for profiling/calibrating monitors (e.g. ColorMunki) – if you have such a device. Buying spectrophotometers at home to evaluate lamps makes no sense, they cost from hundreds to tens of thousands of dollars.
Nevertheless, for the needs of geologists and jewelers produce the simplest spectroscopes based on the diffraction lattice. They cost from 1200 to 2500 rubles. And this is a fun and useful thing.
The spectroscope looks like this :
Choice of LED bulbs. 3. Spectra of light sources
The eyepiece (on the left, where the cone is) must be viewed with the objective lens (on the right) pointing toward the radiation source.
A diffraction grating decomposes light into a spectrum (like a rainbow or optical prism).
Before getting into the spectra of real lamps, let me remind you of general information. (This is covered in some detail at book in the chapter "Quality of Light").
Here I show two spectra of SDLs with an exceptionally high color rendering index of 97 (source here ):
Cold Light :
Choice of LED bulbs. 3. Spectra of light sources
You can see that the color temperature is 5401 K, the index is 97. Most importantly, you can see what colors are visible to the eyes in the spectrum.
Warm Light :
Choice of LED bulbs. 3. Spectra of light sources
Temperature 3046 K, index also 97.
A spectrophotometer – unlike a spectroscope – shows not just which colors make up the spectrum, but also gives their intensity. You can clearly see that the spectra of both lamps contain all the colors that make up white ("every hunter wants to know where the pheasant sits, " i.e., red, orange, yellow, green, blue, blue, violet). The difference in color temperature is achieved by the relative contributions of the cool (blue-blue) and warm (yellow-red) components.
I am forced to mention that this spectroscope is designed for mobile use with my eyes. It is extremely inconvenient to fix the picture, because the eyepiece is small and there are no fixation devices on the camera. So you have to hold the camera with one hand and the spectroscope with the other, and use your voice to control the shooting. You also need to keep the direction of the light source, small deviations from the normal lead to distortion of the colors of the spectrum. Of the nearly a dozen cameras I have at home, the best was the Samsung tablet. The camera there is only 5 megapixels, but good software, and the size and position of the lens on the body of the device allows more or less convenient to fit the spectroscope. The white balance was fixed as "daytime", ISO 400. The images were not processed, just straightened and cropped. The numbers on the right show the color rendering index of the source (100 – daylight in cloudy weather, 99 – incandescent bulb). I’m not very happy with the quality of the pictures – but I couldn’t do any better.
Choice of LED bulbs. 3. Spectra of light sources
So, let’s start from the top down and try to understand what you need to pay attention to in these spectra with concrete examples.
– Daylight and incandescent light : an ideal spectrum in which all the above colors are represented.
– SDLs with a color rendering index of 87 (review here ) and 84 (discussed by in manufacturer’s choice) also show almost a full spectrum. The problem usually becomes the red part – while yellow and orange are usually sufficient, the deep red shades are most often missing. You can’t see them here either. You can also assume (e.g. by the amount of blue in the spectra) that the manufacturers use different 5736SMD LEDs. That is, we are not dealing with the same lamp purchased from different vendors – but with different manufacturers.
– The 78 SDL (see the "Sample Evaluation Test" chapter in the book for a breakdown of it) shows little blue along with the reduced red part. (It may seem that this is not the case compared to the spectrum of the lamp with index 84. But here you have to remember that 84 is a warm lamp, T=2900. And 78 is cold, T=5750 K, there is much more blue by definition). This is the main disadvantage of simple budget LEDs, which form supposedly white light due to blue or magenta LED emission and yellow-orange phosphor light. To the right of blue lies blue – but from the combination described it "does not work". Therefore, in the spectrum of the LED there is usually a dip. Due to this (plus the lack of deep red) the color rendering index drops.
– The lowest spectrum is a high quality compact fluorescent lamp (CFL, T=2700 K, lifetime 12000 hours, the declared color rendering index is not less than 80). And here it is clearly visible, due to what this formally rather high value is achieved. The manufacturer himself calls it "the Tricolor system". That is, it uses a phosphor of 3 components, each of which emits light in the form of a narrow band. (Of course, and such a lamp is not easy to make, because it requires a careful selection of combinations of phosphors). It is the presence of such vertical bands (e.g. purple, green, yellow) – a sign of low-quality light sources. The second consequence of the linear spectrum of the source is the physical absence of some colors in principle (in the picture, for example, there is almost no yellow and very little blue). Obviously, the light of such lamps for the eyes is of little use despite the formally rather high values. Use these lamps in luminaires with quality diffusers (although, of course, the spectrum of the lamp will not change).
Conclusion : The spectra of light sources with a high color rendering index should contain all colors of the spectrum and no intense narrow bands.
Separately I want to warn against haste in the analysis of spectra. In the course of my work I communicated a lot with spectroscopists and I noticed an iron law: the more qualified and professional specialist – the more cautious and evasive in his conclusions he is. From the best of them, professor, head of the spectroscopy laboratory it was generally impossible to get a distinct conclusion (which annoyed me in my youth in the beginning). The eye is undoubtedly the best optical instrument there is. But the analysis and interpretation of spectra is an infinitely complex subject. There are a huge number of different factors at work there. Therefore, I strongly recommend only the simplest qualitative evaluation of spectra with the eyes, without any attempts of cunning reasoning and far-reaching conclusions. It is best to look alternately at the spectrum of the lamp being evaluated and the ideal spectrum of daylight or LN. I.e., a visual comparison between the two.

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