The purpose of this work is to develop a method for assessing the stability of plant development by using the fluctuating asymmetry of the optical density of plant leaves and to design a device for experimental implementation. The paper reviews existing methods and devices for estimating the fluctuating asymmetry by linear and angular dimensions, where the main disadvantages are the high labour inputs and lack of the required functions. It is shown that the optical densities of half-leaves can act as the bilateral traits, by which the stability of plant development can be assessed. The developed method allows to give a comprehensive assessment of energy and ecological compatibility of indoor plant lighting. The hardware component of the device allows to form the vectors of the radiation fluxes incident and transmitted through the leaf for each leaf half in the blue, green and red spectral bands. The experimental model of the device contains an integrating sphere with a photo resistor, measuring the irradiation of the inner surface. Control program is executed on the Arduino Uno microcontroller board based on the Microchip ATmega328P. The measured data are displayed on LCD screen (Keypad Shield) and stored in the external memory (SD card). Colour LEDs are used as the radiation source. The control program provides calibration modes, measurement of optical density and measurement of fluctuating asymmetry of the optical density in three spectral bands. The device was tested on vegetable marrow plants grown in laboratory conditions under light sources with different light quality. The differences in the optical density and fluctuating asymmetry in separate spectral bands were found to be statistically significant for plants grown under different light sources. The designed device provides wide opportunities for rapid assessment of the plant state by the pigment content in leaves. In the presence of appropriate experimental data, it is possible to find a correlation between the values of the optical density of the leaves and other biometric parameters of the plant.