Precision labs series: Ambient light sensors

Welcome to the TI Precision Lab series on Light Sensing. My name is Alex Bhandari-Young. And I am an applications engineer for Texas Instruments optical sensors. This first video is divided into two sections. The first part will discuss the fundamental properties of light relevant to light sensing. The second part will touch on the reasons why one would want to measure light and introduce different types of light sensors.

We see here the electromagnetic spectrum. The electromagnetic spectrum has been categorized by scientists to include a number of different types of radiation, some of which we are familiar with from everyday life. This classification is based on a property called the wavelength. The electromagnetic spectrum includes gamma rays which have the smallest wavelength of less than 100 picometers. This size is less than the diameter of the hydrogen atom.

On the other end of the spectrum are radio waves that range from wavelengths in the millimeters to wavelengths that are larger than the height of buildings. Visible light is a small sliver of the spectrum. The wavelength of visible light is 50 times smaller than the width of human hair.

As electromagnetic radiation is composed of oscillating electric and magnetic fields, in both time and space, the wavelength, in this context, is the distance between peaks of either the electric or magnetic field oscillations. The first plot shows a snapshot of an electromagnetic wave frozen in time and traveling through space, to the right, with the wavelength shown.

The second plot shows a wave frozen in space over a period of time. The period is shown on this plot, and the frequency is equal to the reciprocal of the period. In a vacuum, electromagnetic waves travel at the speed of light C. The frequency is inversely proportional to the wavelength and related by the speed of light C as shown in the equation.

As is shown on the previous slide, wavelength is important in the discussion of both electromagnetic radiation and light because it is how we distinguish between the different types of radiation. Now, we will zoom in on specific regions in the middle of the electromagnetic spectrum. When the wavelength is between 380 nanometers and 750 nanometers, which is about 50 times smaller than the width of human hair, the waves become visible to the naked eye. And this is known as the visible spectrum.

Just below the lowest visible wavelength is the ultraviolet, or UV region. There are many types of UV light, such as UVA, UVB, and UVC. To provide some context, UVA is used in checking currency for counterfeits. UVB, from sunlight, is used in humans and the formation of vitamin D and is also the primary UVA region responsible for sunburn and skin damage. UVC is emitted by germicidal lamps for sterilization.

Just above the highest visible wavelength is the near infrared region. Infrared can be split into near and far IR radiation. Near IR radiation is used by remote controls for televisions. It is also used in night vision cameras that use infrared LEDs to illuminate the scene. Since near IR is just outside the visible region, these devices function without any visible effect that humans can see.

Far IR radiation is emitted by warm objects, such as human beings, and can be seen through thermal scopes and thermal night vision goggles. It is also used in passive IR or PIR sensors, like household motion detectors. The different types of light described can be measured using different types of light sensors. Light sensors sensitive to the visible region are often used for applications requiring display backlight adjustment, such as when your cell phone or car adjusts the brightness of the display in response to changes in ambient lighting.

IR sensors, often when combined with a visible light sensor, can be used for differentiating between sunlight and artificial lighting for day-night detection. UV light sensors are used for measuring the UV index of light from the sun on a given day, as well as in UV-specific applications such as UV sanitization and purification systems.

Wide spectrum light sensors are sensitive to a broad region of the spectrum, including UV, visible, and IR light. These types of sensors can be used in tamper detection applications where differentiating between light sources is not as important as detecting if light is present or not. In this video series, we'll be mostly concerned with visible, wide spectrum, and IR light sensors.

To find more light sensor technical resources and find TI products, please visit the link shown. Thanks for taking the time to watch this video. Please try the following quiz. Question 1, infrared has a higher wavelength than ultraviolet? Recalling the electromagnetic spectrum, infrared wavelengths are above 700 nanometers, while UV wavelengths are below 400 nanometers. Thus, the statement is true.

Question 2, for active and passive IR sensing, which uses near infrared, near IR, and which uses far infrared, far IR radiation, and why? Far IR is used in passive IR sensors, such as motion detectors and thermal imaging. These sensors work passively because far IR is predominantly radiated by humans and warm-blooded animals.

This far IR radiation can be detected or imaged. Active IR sensors need to emit IR radiation, similar to visible light. Near IR radiation is not emitted enough by everyday objects to be sensed. At nighttime, active night vision works by throwing near IR light on the scene. This light is not visible to the human eye but allows a near IR image sensor to see. Similarly, TV remotes contain a near IR LED, and the television contains an IR sensor that can receive the signal.

Question 3, what is the color corresponding to 510 nanometers? Recalling the visible spectrum, we can see that 510 nanometers corresponds to the color green as seen by the human eye. Question 4, for a tablet needing auto display brightness control, what is the best type of sensor to use? Display brightness control requires responding to changes in the visible lighting level as seen by the human eye. So a sensor that measures the visible light level is needed.

Both IR and UV sensors measure different parts of the spectrum from visible light. So these will not work. For example, in a dark environment with a near active IR night vision camera nearby, an IR sensor in a tablet would report a high light level, and the display would go to full brightness. A wide spectrum sensor will measure visible light but will also measure UV and IR radiation. This means the wide spectrum sensor will also have similar difficulties when IR or UV light sources are present. For these reasons, the best sensor to use would be an ambient light sensor.