What Is Photo IC?
Photo IC is a device that incorporates a light-receiving element that receives light and an IC that processes the signals in a single package and is a component with functions that meet the needs of the application.
Photo ICs are classified into monosilic and hybrid types according to their structure. The monosilic type, in which the light-receiving element and the signal-processing circuit are formed on the same chip, does not require wiring between the light-receiving element and the signal-processing and is highly resistant to noise.
The hybrid type consists of a light-receiving element and a signal-processing circuit on independent chips, and they are contained in a single package. Since each chip is connected by wires, the advantage is that the shape of the photosensitive element and spectral response characteristics can be optimized according to the application.
Uses of Photo IC
Photo ICs are used to detect light, and a wide variety of devices exist for different purposes. The main applications of photo ICs are as follows:
- Illuminance meters to measure the brightness of lighting and camera exposure meters
- Receivers for communication using optical fiber
- Encoder modules for detecting movement and rotation angles
- Color sensors that break down into RGB to detect color
- Photo interrupter photodetectors for object detection by light
- Distance measuring device to measure the distance to an object by means of a triangular distance measuring system
Principle of Photo IC
A photo IC consists of a photodiode, phototransistor, or PSD (Position Sensitive Detector) that receives light and generates a current, a circuit that amplifies the output current, and a signal processing circuit that uses the amplified output to process signals. The light-receiving circuit is used to amplify the output current and process the amplified output signal. A wide variety of products combining various photosensors and processing circuits are available depending on the intended use.
There are also photo ICs whose output can be obtained in frequency, although they are special devices. These consist of a photo IC and a current-to-frequency conversion circuit and feature a wide dynamic range.
Photodetectors have different spectral response characteristics depending on the device, but none of them match human visual sensitivity characteristics. The commonly used silicon photodiode has a peak sensitivity in the range of 900 nm to 1,000 nm, but the human visual sensitivity characteristic is in the range of 400 nm to 700 nm, and the peak sensitivity is around 550 nm, so the brightness signal detected by the photodiode and the brightness perceived by humans The brightness signal detected by a photodiode is different from the brightness perceived by humans.
Therefore, for applications that need to match human sensitivity, such as illuminance meters, it is necessary to correct the spectral response characteristics of photodiodes through a visual sensitivity correction filter.
Types of Photo IC
Photo ICs are available with a variety of functions and features. The main types are as follows:
1. Distance-Measuring Photo IC
Photo ICs are used to measure the distance to an object, and PSDs and photosensor arrays are used as light-receiving elements. Combined with a light emitter such as a near-infrared LED, the distance to an object is calculated based on the position of the spotlight projected from the light emitter that is reflected by the object and reaches the photosensor, using the principle of triangular ranging.
2. RGB Color Sensor
This photo IC analyzes the color of incident light by comparing the output of three photo sensors, each of which has an optical filter on its surface that allows only red (R) light to pass through, blue (B) light to pass through, and green (G) light to pass through. The output of each sensor breaks down the incident light into its RGB components, and the main application is in measuring instruments such as color illuminance meters.
3. Illuminance Sensor
These sensors are used to measure the brightness of illumination light, etc. They are photo ICs equipped with light-receiving elements that approximate human spectral response characteristics by using visual sensitivity compensation filters, etc. The main applications are illuminance meters and photographic equipment. It is mainly used in illuminance meters and exposure meters for photographic equipment.
4. Photo Sensor for Optical Communication
This is a device for receiving optical communications using optical fiber. Installed on the end face of an optical fiber, it receives transmitted optical signals and converts them into electrical signals. There are devices with a convex lens in front of the photosensor to improve light collection characteristics.
5. Photoelectric Switch
A photo IC is used to configure a photoelectric switch in combination with a light-emitting device such as an LED. Photo interrupters and photoreceivers of photo reflectors are also included in this category.
6. Photo IC for Encoder
Generally, this photo IC has a 4-channel photodiode array. It is configured to detect the direction and amount of rotation and provides 2-phase digital outputs according to the state of light input to the photodiode array. Combined with a light-emitting element, a photointerrupter with an encoder function can be easily configured.
7. Remote Control Light Receiving IC
This is a receiver IC for remote control using infrared rays and is used in remote control systems widely employed in TVs, recording devices, air conditioners, etc. It is characterized by being covered with resin having visible light cutoff characteristics and having a steep bandpass filter corresponding to the blinking frequency of the transmitter (30kHz to 40kHz). It receives the transmitter’s command signal and outputs it to the processing circuitry of the device itself.