introduction
In 2002, David Berson et al. found a third photoreceptor cell in the retina of mammals. It mainly functions in regulating non-visual biological effects such as human secretion and controlling circadian rhythm. Lighting design also gradually transitions from a single consideration of visual functions to the consideration of both visual and non-visual functions. Studies have shown that dynamic lighting plays a role in the treatment of insomnia, reducing aircraft jet lag, and improving work efficiency.
In order to realize the dynamic lighting design of the LED , it is necessary to control the light color quantity of the light source in real time to modulate the spectrum that meets the requirements of photobiology.
The amount of light color here is a combination of light metrics and color metrics. LED dimming methods commonly used are analog dimming and PWM (Pulse Width Modulation) dimming. The former is a linear adjustment of the LED current, the latter is the use of a switching circuit to change the average of the light output at a frequency sufficiently high relative to the human eye. Preventing color metric offsets is important during dimming. There are two main factors that cause color shift: forward conduction current and PN junction temperature. The color difference produced by analog dimming depends on both, and PWM is primarily determined by the latter. Under normal circumstances, PWM produces a small color difference (white LEDs do not exceed 4SDCM due to junction temperature), and the color difference caused by PWM dimming is not considered in engineering practice.
The PWM driven by constant current has the following characteristics: changing the duty cycle of the LED, the light metric changes linearly and the color metric remains constant. Both the light metric and the color metric are integers that are integer multiples of the average of the square wave period. PWM has also been widely used in engineering practice due to its wide adjustment range.
At present, there is relatively little research on PWM dimming and color grading. Previously, there was a lack of a quantitative calculation scheme that used PWM to simultaneously control the light metric and color metric of the source. Aiming at the above problems, a two-channel PWM dimming color mixing model is proposed, which establishes a one-to-one mapping between the desired light color quantity and the two-channel duty ratio. The algorithm can quantitatively modulate the spectrum of desired luminosity and chromaticity, which provides an effective implementation method for LED dynamic lighting design.
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