In addition, the intensity is not going to be constant through the entire viewing angle, It will tend to be most intense at the center, and will drop off to some degree towards the edges of the viewing angle. So if you have two LEDs with the same wavelength and candela rating but different viewing angles, they should appear equally bright from directly-on, but as you increase your viewing angle, the the one with the narrower viewing angle will appear less bright than the wider one. It also means that the candela rating of an LED is only meaningful within a specified angle. This means that emitting a given amount of light over a small area will produce a higher intensity in candela than the same amount of light emitted over a larger area. The candela in particular is a unit of luminous intensity, which means it indicates a certain amount of luminous flux (that is, radiant energy weighted by wavelength) within a certain solid angle. Photometric measurements must account for angles But for most purposes the numbers will be close enough for a rough idea. Because there are different curves, photometric measurements are only directly comparable if they were obtained using the same luminosity function, and unfortunately the specific function used is not often specified for indicator LEDs (it is more likely to be specified for high power LEDs intended for illumination). These represent different curves established at different times. So comparing candela ratings should be apples to apples, but if you take another look at the graph, note that there is more than one black curve. Note that two different curves are shown: the green curve represents scotopic vision, which is when the eye is adapted for dark conditions, and the black curve represents photopic vision, when the eye is adapted to bright conditions, and more sensitive to color.īecause the candela is a photometric unit, it takes this spectral sensitivity into account-photometric units are based on the luminous flux of a light source, which is determined by multiplying the power spectrum of a source by a luminosity function representing the human eye's spectral sensitivity and then integrating, as opposed to radiometric units, which are based on the total radiant power of a light source, which is simply the integral of the power spectrum. Wikipedia shows the spectral sensitivity for the human eye here: The four different photoreceptors in the human eye have specific sensitivity curves over wavelength, and in aggregate the human eye is more sensitive to green wavelengths than to red or blue. The brightness of a light source depends on its spectrum. The brightness returns to normal if you change the screen or perform another action that eliminates the risks of image retention.Several factors make comparing the brightness of LEDs nontrivial. ![]() When an image is displayed on the screen that may easily cause image retention, the screen brightness gradually reduces in order to protect the TV from image retention.
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