"Optical sensors" is an umbrella term for a more specific set of sensors: ambient light sensors, RGB sensors and proximity sensors. Let’s take a closer look at these cutting edge technologies.
Color Temperature
To understand optical sensors, you need to be familiar with certain parameters. The first is color temperature that is a characterization of light source. This is defined as color ranges produced by a lamp. In physics, the concept of a “black body” is that of an ideal object whose electromagnetic spectrum depends only on its temperature. For instance, at around 2,000 kelvin, green and yellow colors are part of the spectrum, but blue is not. Blue colors need a temperature of around 5,000 kelvin. It seems counter-intuitive that a color is described as “warm” when the temperature is at the low end of the scale (2,000-3,000 kelvin is the most “comfortable” for a human being) and a color is said to be “cold” when its temperature is actually high. Nonetheless, yellow is thought of as warm, but blue is typically associated with cold.
In order to calculate a light temperature, it is necessary to measure the power of red, green and blue. Using matrix calculations and a 3rd order polynomial formula called a McCamy formula can then give you the Correlated Color Temperature (CCT) for your light type. Below are examples of CCTs for different light types.
Brightness
The second key parameter to understand is brightness. When we talk about brightness or light intensity, the unit that is used is a lux. One lux is equal to one lumen per square meter.
A lumen measures the total quantity of visible light emitted by a source. This luminous flux shouldn’t be confused with the power or energy of the light. The power, or radiant flux, includes all electromagnetic waves emitted, unlike the luminous flux that is weighted according to a model of the human eye's sensitivity to various wavelengths.
A lumen itself is equal to one candela steradian. The candela is the SI base unit of luminous intensity. Luminous intensity is analogous to radiant intensity but instead of simply adding up the contribution of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function that is a model of the sensitivity of the human eye to different wavelengths. This is called photopic vision. A second curve, the scotopic curve, can be used for low light conditions but the norm is to use the photopic curve in all conditions.
The steradian is used in 3-D geometry the way that the radian is used to measure angle in two dimensions. It is a measure of how large an object appears to an observer looking from a given point. So, when talking about brightness the following equations apply:
1 Lux
= 1 lumen / m2
= one candela . Steradian / m2
As points of reference, 1 lux is the light of a full moon on a clear night, a few hundred lux is the light from an office building. An overcast day might have the strength of 100 lux. 400 lux is your standard sunrise or sunset, 10,000 lux is full daylight, direct sunlight is even more.
Vishay markets their ambient light sensors, RGB sensors and proximity sensors based on optical technology.
The VEML6030 from Vishay is a highly accurate ambient light sensor. It includes a high-sensitivity photodiode, a low noise amplifier and an ADC, which allows for the output value to be read through an I2C. This is called a silicon eye because the response is very close to that of a human eye.
The VEML6040 is an RGB sensor. It measures the power received for each red, green and blue wavelength. The die is covered by different filters in order to detect the various colors. Thanks to RGB colors measured by the VEML6040, CCTs can be easily calculated using McCamy formulas.
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Finally, the VCNL4020 is an infrared proximity sensor. This sensor does not work with visible light as it emits infrared wavelengths and measures the reflected power. As soon as a material approaches the sensor, the reflected power changes. Depending on material and distance, the reflected power is different and different thresholds can be programmed to fit proximity sensor applications.
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Optical sensors are a powerful tool for engineers to have in their solution portfolio. As the sensor revolution marches on, optical sensors will become more and more standard for certain applications.