Analog Circuit of Light Detector for CMOS Image Sensor

Optical Sensor in Machine Vision for Agriculture Application

Optical crop sensors assess crop health conditions by shining light of specific wavelengths at crop leave and measuring the type and intensity of the light wavelengths reflected back to the sensors. Not all optical sensors use the same light wavelengths. Different color light waves can be used to measure different plant properties. Commercially available crop sensors use two or more of red, green, blue or near infrared (NIR) color light waves. Example of usage of a typical optical crop sensor is shown in Figure 1. The optical sensor employs two types of light sources which are Near Infra-Red and Red. Even though some of the optical sensors (Bragagnolo et al., 2013) employ xenon transmitter as the light sources, the photodetector is the common device for the reflected light detection devices.

Figure 1.

A typical of optical crop sensor application

The leaf color indicates the level of nitrogen required by the plants. A dedicated chlorophyll meter is very expensive (Bragagnolo et al., 2013) due to multiple light source (LEDs) requirements. The single wavelength based crop sensor is recently investigated (Intaravanne & Sumriddetchkajorn, 2015) which is based on android device. This work employs CMOS Image sensor chip (camera chip) as the photodetector. Even though it is simple, it cannot be used for in more advance horticulture or greenhouse application.

Figure 2 shows a state of the art of the electronic part (Front-End) of the optical crop sensor (Biggs et al., 2002). Normally, the optical crop sensor used the constant light source, as this offer the simplest design. The amplifier is, therefore, a standard CMOS operational amplifier. The amplifier is used to amplify small voltage which is converted from light energy using the photodetector. The Front-End topology inherits low sensitivity due to the high dark current of the photodiode (when there is no light) and noisy CMOS operational amplifier. A very sensitive optical crop sensor indicates that it can detect dim light which means the light source power consumption can be reduced, thus leads to the potential of a portable system. The CMOS operational amplifier gain should be high when the sensitivity is low, this, however, will incur high power consumption to the CMOS operational amplifier. For low power CMOS operational amplifier, a sensitive photodetector is therefore required. It is also a requirement to have low noise CMOS operational amplifier in order not to affect the overall sensitivity.

Figure 2.

Electronic parts and the detectors

An active multispectral imager whereby a monochrome Charge Coupled Device (CCD) camera (5 MPix) is mounted in a position two meters above the canopy surface inside a box with a LED light panel also inside the box illuminating the surface to produce nine spectral has been developed (Pajares et al., 2016). However, it is uncertain whether the design can eliminate the unwanted reflected light.