A far lower lumen rated flashlight, which is efficiently focused, can appear brighter and throw further than a less efficiently focused higher lumen rated torch. From this, you may think about something of the light output measurement and perceived brightness. So what's the relationship between them?
Perceived brightness, in other words, is how your eyes observe, evaluate, and process the illuminance that you see. As for light output, it can be measured in a few different ways. For example, lumens are the measure of the total light emitted by a LED. Lux measures the amount of light that strikes a point on a wall. 1 lux is equal to 1 lumen per square meter. And Candela is the intensity, a measurement of how far a light will shine. Taken at a distance of 1 meter, a reading of 1 lux indicates 1 candela, at 10 meters, a measurement of 1 lux would indicate 1 x 10² = 100 candela. Throw is derived directly from candela. An actual distance for throw can be calculated from the candela value and will be given in meters.
And human eyes don't perceive changes in light level in a linear manner and slight variations in perception. If the light level is dimmed to a level of 50% as measured by instrument, our eyes don't perceive this light level to be 50% of the original level. Besides, increase or decrease of light output would not yield a noticeable difference in perceived brightness. For example, a 170 lumen flashlight and a 200 lumen flashlight will appear to be approximately the same brightness. While, a 10 lumen flashlight would be noticeably brighter than a 5 lumen flashlight.
There are studies have shown that the eye perceives light in a logarithmic manner, mathematically speaking, in an approximately squared power relationship. The approximate perceived light is proportional to the square root of the measured brightness. If the measured light level is 10% of the max light level, then the perceived light level is √0.1=0.316 or 31.6% of the max light level, as it shows in graph below.
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Perceived brightness, in other words, is how your eyes observe, evaluate, and process the illuminance that you see. As for light output, it can be measured in a few different ways. For example, lumens are the measure of the total light emitted by a LED. Lux measures the amount of light that strikes a point on a wall. 1 lux is equal to 1 lumen per square meter. And Candela is the intensity, a measurement of how far a light will shine. Taken at a distance of 1 meter, a reading of 1 lux indicates 1 candela, at 10 meters, a measurement of 1 lux would indicate 1 x 10² = 100 candela. Throw is derived directly from candela. An actual distance for throw can be calculated from the candela value and will be given in meters.
And human eyes don't perceive changes in light level in a linear manner and slight variations in perception. If the light level is dimmed to a level of 50% as measured by instrument, our eyes don't perceive this light level to be 50% of the original level. Besides, increase or decrease of light output would not yield a noticeable difference in perceived brightness. For example, a 170 lumen flashlight and a 200 lumen flashlight will appear to be approximately the same brightness. While, a 10 lumen flashlight would be noticeably brighter than a 5 lumen flashlight.
There are studies have shown that the eye perceives light in a logarithmic manner, mathematically speaking, in an approximately squared power relationship. The approximate perceived light is proportional to the square root of the measured brightness. If the measured light level is 10% of the max light level, then the perceived light level is √0.1=0.316 or 31.6% of the max light level, as it shows in graph below.
