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.

In this video, we will discuss the common measurement units encountered in light sensing. Five to 10 years back, incandescent bulbs were the only common type of household light bulb. If you were looking to buy a light bulb for your house, you would use the electrical power spec in watts printed on the box to tell how bright the bulb would be.

Now as different types of light source technologies have become common and more efficient bulbs produced, light bulb manufacturers cannot just use watts to indicate bulb brightness. For example, a typical 60 watt incandescent light bulb outputs a similar intensity of light as a typical 15 watt fluorescent bulb. This is because the watt ratings given for light bulbs are speced in terms of electrical input power rather than optical output power.

Optical output power, as seen by the human eye, is measured in lumens, which is now displayed on most light bulb packaging as a way to accurately compare the brightness of different bulbs. The 60 watt incandescent and 15 watt compact fluorescent bulbs both output between 800 and 900 lumens. Now we will talk about lumens in more detail.

The lumen is a measure of the total visible light emitted by a light source. To measure lumens, one could place the light bulb in a sphere and measure the brightness of light emitted by the bulb from all directions, as shown. We can also see from the graphic that changing the size of the sphere does not affect the measurement because all light is captured regardless of the size of the sphere.

Lumens is a measure of the light seen by the human eye and only measures the visible region. Using the lumen, the total visible light output of different light sources can be compared.

Now consider placing a circular photodetector at the surface of the sphere as shown. For different size spheres and the same size photodetector, we can see that the photodetector will collect less light per unit area the farther it is from the source. Since lumens is a measure of total light and the photodetector can only detect the light that reaches it, we cannot use lumens as the detector output units.

Lux builds on the lumens measurement by restricting it to a certain area. Lux is a measure of visible light falling on a given area and is equivalent to lumens per meter squared. Lux allows us to measure light that strikes a surface. In an outdoor or office environment, the total lumens emitted by the sun or by an artificial bulb is not as important as the light reaching an area of interest. For this reason, lux is a common output unit for visible light sensors.

The table shown gives example lux levels for situations commonly encountered in daily life to put the unit of lux in perspective. Common light sources, including light bulbs, do not emit light uniformly in all directions. This includes the light bulb shown in the sphere we have been examining. Let's consider a cone placed inside the sphere with an angle theta, as shown. It can be seen that the lumens inside the cone would be unchanged with the size of the sphere.

However, due to the nonuniformity of the light source, when the same quantity is measured from different directions, it will vary. The solid angle subtended by the cone is measured in steradians and is related to the angle theta by the formula shown.

Candela is a measurement defined as the amount of lumens contained per unit steradian. the candela is particularly useful in characterizing directional properties of light sources.

The candela was originally based on the light emitted by a candle, though the definition has since evolved. The typical candle still emits roughly one candela of light in most directions.

The last type of scenario I will cover involves changing the size of the light source. Consider the two sources shown. Both produce the same lumens and candelas, but one is larger than the other. In this case, the smaller one will look much brighter to the human eye because the same amount of light is emitted from a smaller area.

Nits is a unit used to measure the per unit area of brightness of light sources. It is defined as candela per unit surface area of emission. In an office environment, this measure is important because larger light sources are easier to look at whereas smaller light sources look brighter and can be hard to look at directly.

This is the reason for having lamp shades and diffusers on light sources to spread the light out over a larger emitting area, making it easier on the eye. It's also quite common to see nits as a unit of measurement for mobile and laptop display brightness. A typical mobile display has around 700 minutes of brightness.

Here we summarize all quantities seen so far and also introduce the name of each quantity. Luminous flux is measured in lumens and describes the total light emitted. Illuminance is measured in lux and describes the light incident on a surface per unit area. Luminous intensity is measured in candles and describes light in a direction. Candela has units of lumens per steradian. Luminance is measured in nits and describes light in a direction per emitter area.

So far, we have talked about photometric quantities, which measure intensity perceived by the human eye and take into account the human eye response. We will now introduce radiometric quantities. Radiometric quantities measure power of electromagnetic radiation in watts. This spans across the entire electromagnetic spectrum, including visible light, making them a superset of the photometric quantities.

While photometric quantities incorporate the human eye response, the radiometric quantities have no weighting for different wavelengths of radiation. We will now introduce the radiometric counterparts of the photometric quantities seen so far.

We see the photometric units described so far in the first two columns. The next two columns give the radiometric analog for each unit. The description column at the end provides a general description of the row. Because the description refers to power instead of visible light, it describes the radiometric units.

Radiometric units use the watt for radiant flux, which is the total power emitted. This is essentially the only change between photometric and radiometric quantities. Instead of using the lumen to measure output visible light, radiometric uses the watt so that all radiation is weighted equally. Then all other units can be updated by replacing the lumen with a watt.

In previous videos, we have worked with normalized spectral plots for emitted and received light. Now we introduce the units used for these plots. This plot shows the intensity of light from the sun as observed on the surface. The blue curve corresponds to a surface placed outside Earth's atmosphere. The green curve corresponds to a surface within Earth's atmosphere.

Since we measure sunlight on a surface, the radiometric quantity of a irradiance is used. As expected, the units consist of watts per meter squared. However, there is an additional per nanometer term in the denominator. Let's see why this is the case.

In this example, we take the plot of sunlight in space and zoom in on the 400 to 1,100 nanometer region. Consider measuring the spectrum at 50 nanometer steps. We could use a spectrometer to split the light into groups, each with 50 nanometer bandwidth, and then measure the power output for each group.

Our readings would be of the irradiance measured for each band in watts per meter squared. This result is shown in blue. We now have the irradiance every 50 nanometers emitted by the light source. However, if we choose to retake our readings with 20 nanometer bandwidth, then the orange plot is the result. Note that now the irradiance in each group has decreased by a factor of 2.5 because each group contains fewer wavelengths.

To make the plot independent of the bandwidth of measurement, we need to divide by the bandwidth. With these new units of spectral irradiance, plots measured with different spectral bandwidths will now equate. Any measurement taken from the plot needs to also specify an associated bandwidth.

Using the sun's spectrum we cannot pick the irradiance at 1800 nanometers directly. We need to pick a bandwidth, say 1799 to 1801 nanometers. And this irradiance measure is computed by taking the integral of the curve from 1799 to 1801 nanometers. This gives back a value in irradiance units of watts per meter squared corresponding to the wavelengths from 1799 to 1801 nanometers. This is generalized in the formula shown.

Now that we have covered both photometric and radiometric units, we will show how to convert between these two units. We may do this by using the spectral response of the human eye to relate watts and lumens. At the peak of 555 nanometers on the photopic region, one watt is 683 lumens.

Using this data point and the photopic curve seen previously, we can plot the lumens per watt for each visible wavelength as shown. The y-axis of this plot shows how many lumens the human eye sees for a watt of power at each visible wavelength.

Typical laser pointers have a power rating of around 10 milliwatts. And for this example, we consider a 10 milliwatt 650 nanometer red laser pointer and a 10 milliwatt 520 nanometer green laser pointer. A laser shined on a high reflectivity white wall will illuminate the room like a point source as shown.

Using the plot, we can calculate how many lumens each laser dot will emit into the room as shown. We can see that the green laser dot emits about 2.7 times more lumens into the room than the red laser light. And this is due to the photopiv response at 650 and 520 nanometers. This is why green lasers appear brighter than red or blue lasers of similar radiometric power. 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. What is the best unit to specify brightness of common light bulbs in? The correct answer is lumens.

Question 2. Which the following is an expected value of brightness for typical indoor lighting? The correct answer is 700 lux.

Question 3. A 500 nanometer green light source with irradiance of 1,000 milliwatts per steradian is shining on an ambient light sensor with photopic response. What will the light sensor read?

The correct answer is not enough information is provided. We discussed how placing a photodiode at different distances from a light source will affect the reading so distance from the source is needed.

Question 4. A 500 nanometer green light source with a radiance of 1,000 milliwatts per steradian is shining on an ambient light sensor with photopic response. The distance between the LED and light sensor is one meter. What will the light sensor read?

Using the lumens per watt plot from the slides, watts per steradian can be converted to lumens per steradian. Then the distance can be used to convert candela to lux as shown.

Question 5. An 850 nanometer IR LED with radiant flux of one watt is shining on an ambient light sensor with photopic response. What will the light sensor read?

Because ambient light sensors only respond to light in the visible region, the sensor will read zero lux.