![]() Light measurement units - Lumen, Lux and Candela. The most common units used to measure light are Lumen, Lux and Candela. We can measure light based on the brightness seen by the human eye. Some of the highest candela ratings are seen in spotlights or lasers since most of its light is concentrated in one direction. The light from one candle is roughly one candela. A candela measures the amount of luminance intensity from any point in a single direction. Candela: The unit measurement for light intensity.The candela is the SI unit for luminous intensity. ![]() Luminous intensity: Amount of visible light emitted from a source in a particular direction per unit solid angle.Luminous Flux (Lux): The amount of light emitted in every direction and measured in lumens.Lumen: The total amount of visible light emitted from a light source.Luminance is also referred to as brightness. Luminance: The perceived amount of light emitted from an object.Illuminance: The amount of light that hits a surface.As I mentioned earlier, HDR and WCG are actually different topics, but they are almost always spoken of together when discussing HDR in general.Before we compare light measuring units, it's essential to understand some concepts: We’ll be looking at how all of this impacts HDR in our next discussion, where we’ll also talk about colorimetry and wide color gamut. Note that we’ve only discussed measurement of total intensity here, not colorimetry. So there you have it-the fascinating world of light measurement. To give you a sense of perspective, a sunny day will measure somewhere around 10,000–12,000 nits and a moonlit night, with the moon directly overhead, will measure in the very low single (or even fractional) digits. Why do you care? Because if you try to look up the light levels for bright daylight or the moonless night sky, you’ll most likely get values in lux (approximately 25,000 lx and 0.002 lx respectively).Īnd finally, we come to the final metric that we need to understand: Nits! The scientific name for this unit is “candela per square meter” (cd/m), but for some reason, the term “Nit” (nt) has become widely adopted when talking about displays (or screens)-the two terms are interchangeable. If you had the same 1 lumen over an area of 2 square meters, you would only have 0.5 lux and it would appear half as bright. 1 lux is simply 1 lumen per square meter. This one is very simple: it defines how much luminous flux is spread over a given area. The next term in our “dictionary of light” is the lux (lx). Why do you care? Because most light sources are commonly labelled with their output in terms of lumens. 1 lumen is defined as 1 cd multiplied by the steradian of the beam for which we’re making the measurement (a sphere has a solid angle of 4P steradians, so a source delivering a uniform 1cd in all directions is emitting 12.57 lumens). Luminous flux is measured in lumens (lm). In other words, it doesn’t include the infrared or ultraviolet spectra, and other wavelengths are weighted by the eye’s sensitivity. This quantity is called “luminous flux” and is analogous to “radiant flux”-the measurement of the total power of electromagnetic radiation-except that it takes into account the eye’s variance in sensitivity to different wavelengths of light. ![]() So now we have to consider the measurement of the perceived power (by the human eye) of light.
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