As automobiles become more complex and more electronically driven, it's no surprise that more innovative and unique technologies are getting implemented in this space. One application increasing in the automotive environment is the proliferation of ambient light sensors throughout the passenger cabin. This article discusses the fundamentals of ambient light sensing and the advantages of such devices in the automotive uses.
Light sensor overview
Before we jump into the details, let's take a moment and review some fundamentals on light and sensing it. First of all, when we talk about light, we are referring to the visible portion of the electromagnetic spectrum, which defines the entire range of electromagnetic radiation. This spectrum usually is divided into seven sections, from the longest wavelengths to the shortest: Radio, microwave, infrared (750 nm-1 mm), visible (380 to 770 nm), ultraviolet (10 to 380nm), x-ray, and gamma ray radiation. When sensing ambient light, care must be taken to attenuate unwanted infrared (IR) and ultraviolet (UV) lightboth these spectra are not visible to the human eye and can cause inaccurate ambient light sensor readings.
In this spectrum, the human eye is most sensitive from around 450 to 650 nm, with peak sensitivity around 550 nm (see below)important to realize when choosing an appropriate ambient light sensor. Several sensors offer very broad spectral response, which includes unwanted IR sensitivity.
For clarification, when we say ambient light we are referring to the surrounding light or environmental light that is everywhere equally intense and has no directionality. A beam of directional light, therefore is not ambient light.
Detecting light can be done several ways, for example by using a phototransistor, photoresistor, or photodiode. For overall light sensing requirements in today's applications, monolithic IC-based photodiodes are the light sensor of choice. A photodiode is a semiconductor device used to detect light and generate an electrical current, constructed from a single crystal silicon wafer, similar to those used in the manufacture of integrated circuits.