![]() The yellow light is similar to candlelight, relatively uniform in all directions. In the diagram the two light sources, red and yellow cover an equal area of space. For a diffused light, for example, a candle, moving away from 0° will reduce the luminous intensity at a lower rate, if at all. If the light source is directed, for example, an LED flashlight, and the direction straight ahead along the beam is taken for 0°, then moving away towards 180° will greatly reduce the luminous intensity. The value of this function is the highest for the light of a wavelength of about 550 nanometers, which is a green light, and lower for longer or shorter wavelengths.Ĭhanges of an angle in the plane where the light is distributed also change the luminous intensity, as shown in the diagram. It is easy to see from the formula for luminous intensity: it depends on the luminosity function, which in turn is different for different wavelengths. This is because the luminous intensity is adjusted based on the way different wavelength light is perceived by people. Luminous intensity is considered to be a unit of power, but it is different from conventional power because it varies not only depending on the energy of the source, but also on the wavelength. In this case, the latter will appear brighter, even though their luminous intensity is the same. Two objects with the same luminous intensity can also appear to have different brightness if, for example, one is diffused light and one is a point light. Different people can think of the same light to have varying brightness, even though its luminous intensity did not change. Brightness is also generally not perceived at a specific angle but evaluated more holistically. It is different from brightness, which is a subjective value, as perceived by the human eye. Luminous intensity is a physical characteristic of light. It considers only the focused light and not the light dispersed in all directions. Luminous intensity measures the power of a light source for a given unit solid angle. Measuring Luminous Intensity and Distribution I just thought it was easier to have the candela as the slider, because that’s the number you get when you measure with your lux meter at 1m distance, and the calculator does the conversion to cd/mm^2 for you when you put in the die area from the datasheet.Luminous Intensity in Museum Conservation Projects If you want to do other LEDs that don’t have lambertian distribution you need to take a screenshot of the datasheet relative intensity graph, overlay it on the desmos calculator with some transparency, adjust the size so that it matches, then use trial and error to modify the cos() function until the orange line matches the image from the datasheet.įor tests done by people like koef3 and others, usually they list luminance (cd/mm^2) instead of directly candela, so you can just manually change the luminance to a number and then change the intensity to “l*s” and then you can adjust the luminance slider instead. ![]() ![]() If we do the CSLNM1.TG, we get 324.5 lumens from the calculator, again very close to the rated 325 lumens. ![]() Putting that into the calculator then gives 514 lumens, extremely close to the 515 spec. Here I used cos(1.025*angle) instead of just cos(angle) in order to make the function match the datasheet more closely: If we put 171cd intensity and 1.59*1.25 = 1.9875mm^2 die size we get 86cd/mm^2 and 537 total lumens.Īs I mentioned earlier this is a slight overestimate because of what you can see in the first image.Ĭorrecting for non-lambertian distribution: Here you can see that flat LEDs have almost a perfect Lambertian distribution (orange line), although the intensity drops slightly faster at large angles near 90 degrees, so the lumen numbers given by the calculator are a slight overestimate.īy setting the intensity (candela) and LED die size based on test or datasheet info, the luminance is calculated (cd/mm^2) and then you can adjust the min and max angles of light collection.įor example, the CSLPM1.TG has 171cd and 515 lumens at spec. If the LED does not follow the Lambertian distribution, a special function needs to be used instead of cos(). These LEDs have an intensity that follows Lambertian distribution, which means that the intensity of the LED is proportional to cos(angle), making it very simple to put inside the calculator. (but it can do other LEDs if you put in some effort) This works only for no-dome LEDs such as the black flat, white flat, or CFT90.
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