Welcome to the TI Precision Lab series on light sensors. My name is Alex Bhandari-Young. And I'm an applications engineer at Texas Instruments. While the common types of light sources that we encounter in daily life may look similar to our eyes, such as fluorescent and LED light bulbs both appearing white, they can actually be quite different.

In this video, I will discuss some common mechanisms by which light is generated and the different light sources that operate based on these mechanisms. I will also use the concept of wavelength, introduced previously, to cover how these light sources differ in terms of the wavelength of light they emit.

Any object that has a temperature above absolute zero will radiate electromagnetic waves of several wavelengths due to a process called thermal radiation. As was mentioned in the previous video, the human body radiates predominantly in the far infrared region.

As the temperature increases to around 525 Celsius, which is 798 Kelvin, the thermal emission will start to enter the visible region and appear to the human eye as a red glow in a process called incandescence. Further increasing the temperature causes the color of light emitted to shift to smaller wavelengths as shown on the right.

Thermal radiation is the process responsible for light emitted by the sun as well as by incandescent and halogen light bulbs. The first light source we will explore is the sun. The sun has a surface temperature of about 5,778 Kelvin. The plot on the right shows the spectral emission of the sun zoomed in on the visible region.

Spectral plots have wavelength as the x-axis and show for each wavelength how much power is radiated. In this case, the y-axis is normalized so power is shown relative to the peak. It can be seen that sunlight is comprised of a number of wavelengths of visible light as well as other types of electromagnetic radiation that we cannot see.

As the combination of all these wavelengths radiated is referred to as a spectrum, the plot shown is referred to as a spectral plot. Sunlight in space is most intense between 470 and 510 nanometers, which consists of blue and green colors.

The sunlight we see on Earth must pass through the atmosphere. And this has an effect on the light that makes it through. Shown in blue is the same sunlight spectrum in outer space from the previous slide but now zoomed out to show a larger range of wavelengths. Shown in green is the light that makes it to sea level. Sunlight is absorbed at certain wavelengths by different molecules in the atmosphere, such as water, carbon dioxide, oxygen, and ozone, as shown.

Here is the zoomed in view of the spectrum of sunlight that makes it through the atmosphere and down to sea level. We can see that the intensity of light is now quite flat within the visible region.

Household incandescent light bulbs consist of a tungsten filament in an inert gas. Electrical current is passed through the filament, which heats to a typical temperature between 2,000 and 3,000 degrees Kelvin. Similar to the sun, the heated filament radiates a spectrum of different wavelengths, including visible light, as seen in the spectral plot on the right. The peak for incandescent sources is typically in the infrared region due to the low temperature. And this results in a more yellow light compared with sunlight.

Halogen light bulbs replace the inert gas in incandescent bulbs with a halogen gas and typically operate at temperatures over 2,700 degrees Kelvin. This produces a spectrum that is slightly shifted to the left, containing more of the lower wavelengths of the visible region. These lower wavelengths are more blue in color and shift the yellow color slightly closer to white.

The three thermal sources just described have similar emission spectra characterized by a smooth curve. The main differences in spectra were due to shifts because the sources are at different temperatures. Fluorescent and LED light sources operate using different processes.

Fluorescence is a process by which electrons in an atom absorb a shorter wavelength of light and emit a photon of a longer wavelength of light. This is illustrated in the diagram shown. Fluorescent light bulbs consist of mercury vapor inside a tube coated with a fluorescent material and use this process to convert UV light produced by the mercury to visible light.

Because this process is governed by electrons absorbing specific wavelengths of light and emitting other specific wavelengths of light, the spectral output of fluorescent bulbs has distinct peaks at these emission wavelengths as shown in this spectral plot.

The final light source we will discuss is the light emitting diode or LED. LED light sources generate light as electrons pass through the band gap of the PN junction in a diode. There are many different types of LEDs for different wavelengths of light. White LEDs are actually a combination of several LEDs. The combination of these different spectra from each LCD is chosen to produce an overall white light color. An example spectrum of this is shown on the screen.

There are also near-infrared LEDs that are not visible to human vision. These LEDs are used in near-infrared applications such as TV remotes and night vision cameras as mentioned in the first video of the series.

Understanding the spectrums of these different types of light sources is important when we consider real world light sensing scenarios. Shown is an indoor environment where a light sensor may be used for controlling the indoor lighting level or in a product such as a thermostat, cell phone, or watch for display brightness adjustment.

Indoors, the light sensor may be placed under any of the previously described light sources or any combination of the described light sources. In this example, there is sunlight shining through the window in addition to indoor lighting consisting of both LED and incandescent types. Between daytime and nighttime, this combined spectrum will also change.

The same is true for other applications such as display brightness and headlight control in vehicles as well as outdoor applications. How a spectrum influences both the human eye and light sensors will be discussed in the next video.

To find more light sensor technical resources and search TI products, please visit the link shown. Thanks for taking the time to watch this video. Please try the following quiz.

Question 1. At what approximate temperature does light emitted by a hot object start to become visible? The answer is 525 Celsius, which is approximately 800 Kelvin.

Question 2. The mechanism of incandescent bulbs is very similar to blank except that the temperature is lower. The operational mechanism of incandescent bulbs and halogen bulbs is both thermal heating of a filament. LED and compact fluorescent bulbs work on different principles so the correct answer is C.

Question 3. For sunlight passing through the atmosphere, select all of the following that contribute to the predominant atmospheric absorption. Of the options, water, H2O, carbon dioxide, CO2, and ozone, O3, all contribute predominantly to the atmospheric absorption as can be seen in the plot.

Question 4. In a brightly lit office space with a combination of natural and artificial light, the spectral content of the light is the sum of the spectra of the individual light sources. Please also provide an explanation.

This is true. While the relative spectral plots we are showing in this video cannot be directly added because they are normalized and so do not take into account differences in intensity between light sources, the actual spectral plots are measured in units of power per area on the y-axis. These plots can be directly added for each wavelength on the x-axis to create a combined spectral plot for both light sources. The non-normalized spectral plots will be introduced after we discuss units for measuring light in future videos.