1. Working Principle  

Two LED light sources emit two types of light: red light (650 nm) and infrared light (940 nm). Both types of light penetrate the leaves and strike the receiver. The light signals are converted into analog signals, which are amplified by an amplifier, then converted into digital signals by an analog-to-digital converter. The digital signals are processed by a microprocessor to calculate the SPAD value, which is displayed on the LCD screen.

2. Calibration and Calculation of Measurement Values  

(1) Calibration and Calculation of Measurement Values: The two LEDs emit light in sequence. The received light is converted into an electrical signal, and the ratio of light intensity is used for calculation.  

(2) After the sample is clamped by the pressure head, the two LEDs emit light again. The light transmitted through the leaf hits the receiver, is converted into an electrical signal, and the ratio of transmitted light intensity is calculated.

(3) The values from steps 1 and 2 are used to calculate the SPAD measurement value, which represents the current relative chlorophyll content of the clamped sample leaf.