Ambient Light Sensor
Course: Advanced Design Support
Authors: Karim de Waard en Fleur van Midwoud, Group 09
This article is about Ambient Light Sensors. Ambient light sensors are used to detect light or brightness in a similar way as the human eye. They are used wherever the settings of a system have to be adjusted to the ambient light conditions as perceived by humans.
What are the typical application fields, domains and products using this technology?
Products that use ambient light sensors can be divided in five categories as result of their purpose, these purposes are:
- Saving battery power
These sensors are used as a low cost power saving solution for hand-held electronic devices. Examples are: PDAs, mobile phones and notebook PCs.
- Saving energy
These sensors can help saving energy by dimming lamps of office buildings, exterior lightings and traffic signals
- Automatic dimming of flat panel displays
These sensors are used in LCD screens and their goal is to maintain the same display appearance under all light conditions.
- Automatic dimming of instruments in automobiles to ensure reliable visibility under all circumstances
- Headlamp control in cars improves road safety by automatically turning on the light in twilight or by entering a tunnel
- Saving battery power
What is the working principle?
An Ambient Light Sensor is a specific version of a photodiode. A photodiode is a type of photo detector capable of converting light into a voltage or current, this depends on the mode of operation. A photodiode is a PN junction or PIN structure, the figure explains the working principle of a PN photodiode. Lights enters the top of the photodiode, a thin top P-type layer allows most photons to pass into the depletion region where electron-hole pairs are formed. The electric field across the depletion region due to the built in diode potential causes electrons to be swept into the N-layer, holes into the P-layer. Actually electron-hole pairs may be formed in any of the semiconductor regions. However, those formed in the depletion region are most likely to be separated into the respective N and P-regions. Many of the electron-hole pairs formed in the P and N-regions recombine. Only a few do so in the depletion region. Thus, a few electron-hole pairs in the N and P-regions, and most in the depletion region contribute to photocurrent, that current resulting from light falling on the photodiode.
The special thing about Ambient Light Sensors is that they perceive brightness in the same way as human eyes do. This can be illustrated with Figure 4. It shows the spectral emission of different light sources compared to the spectral sensitivity of human eye. Standard photo detectors have a spectral response ranging from 1100nm right down to 350nm with the peak sensitivity around 880nm. Human eyes, however, detect a much narrower wavelength range, namely from 400 nm to 700 nm with the peak sensitivity at 560nm.
Ambient Light Sensors "work" in the same area as the human eye, therefore they provide a much better accuracy for the brightness measurement. The results can be found in Figure 5, it shows the signals received by a standard light sensor detector and the OSRAM ambient light sensor SFH 3410.
Inputs and outputs of the technology
What an Ambient Light Sensor basically does is converting light to a voltage or current. The response of an ambient light sensor should be linear. Photodiode output typically requires amplification while phototransistor output may not.
Due to the manufacturing process ambient light sensors of different production lots will yield different outputs for the same illuminance. The magnitude of this sensitivity variation depends on the sensor type. To account for this, some ambient light sensors are offered in defined sensitivity bins. These are described in the datasheets and in the application notes of the respective sensors. The sensitivity variation can also be overcome by calibrating the assembled unit in the production line.
We made an abstract prototype of a product using an Ambient Light Sensor, it can be found on YouTube:
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 Vishay's Ambient Light Sensors.
 Ambient Light Sensor in computer.
 Photodiode: Schematic symbol and cross section.
 Spectral emission of different light sources compared to the spectral sensitivity of human eye (V lambda).
 Signals received by a standard light sensor detector and the ambient light sensor SFH 3410.
 Output signal Ipce of OSRAM SFH 3410 and OSRAM SFH 3710 versus Illuminance.
 OSRAM Ambient Light Sensors SFH3410, SFH3710.
 Photodiode Wikipedia.
 Dr. Christine Rüth, Andreas Vogler, Wilhelm Karsten, General appnote for Ambient Light Sensors.
 Vishay Ambient Light Sensors.
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