The irradiance experiment that follows features an Ocean Optics Flame-S spectrometer and was designed and written by Professor Kamran Golestaneh of the Chemistry Dept. at Mt. San Antonio College in Walnut, Calif. It appears here with his permission.
“In a nutshell, the experiment is about a simple and effective method to measure the irradiance and luminance of different common screw-base light bulbs as well as an advanced Phillips ceramic metal halide (CMH) bulb, which is the latest in efficient technology for indoor plant growth,” said Professor Golestaneh. “My expertise is to make complex matters simple for my students to process while learning the theory in the context of experimentation.”
Note: The material here has been edited for length and clarity; some lab exercises and sample assignments are included.
Light: Sources, Spectral Efficiency, Irradiance and Illuminance using an Ocean Optics Flame-S Miniature Spectrometer
The purpose of this experiment is to use a simple and convenient method to analyze different sources of light using an Ocean Optics Flame-S spectrometer, evaluating the visible light of each source for its absolute irradiance spectra as well as luminous power efficiency and irradiance.
Light is a form of energy radiation that propagates as an electromagnetic wave or a stream of packets of light called photons. Think of each photon of light as 1 wave having a wavelength (λ), frequency (n) and energy (E) according to the relationship E=h.n, where h = 6.626 x 10-34 J.s/photon (Planck’s constant) and n is the frequency of light in cycles/second or Hertz (Hz). Therefore, E is the energy of 1 photon (J/photon).
Frequency is related to wavelength according to n=h.c/λ, where c = 2.998x108 m/s (speed of light in vacuum). Therefore, we can calculate the power (energy/time) of a stream of photons as a function of either wavelength or frequency. Power is measured in Watts (W), which is same as Joules (J) per second (J/s).
Light sources assist our vision. Limits of visible light detection depend on our brain’s perception of color but typically span over a wavelength range of 380 nm (deep violet) to 780 nm (dark red).
Visible light is a narrow band of the electromagnetic (EM) spectrum as shown above. White light (e.g., sunlight) is a continuous spectrum of visible light comprising Red-Orange-Yellow-Green-Blue-Violet (remember ROYGBV colors). We can also “estimate” the color white by mixing the primary colors RYB (Red-Yellow-Blue) or mix RGB (Red-Green-Blue) as commonly done in computers, TV or projector screens. These three-color combinations mimic the white response in our vision.
Three Cones and Three Colors (RGB)
Our vision utilizes three types of color receptors (referred to as “cone cells” or “cones”), which are sensitive to Red, Green and Blue (RGB) colors; only their relative response determines our range of color perception. Humans have trichromatic (3-color) vision as opposed to some animal species, which have tetrachromatic vision (RGB colors + UV light cones).
Note that there is wide overlap among these receptors, especially the red and green cones. Therefore, color response is highly dependent on the relative intensity response of each cone.
Let’s start by defining white. White is not a color. White is the presence of all visible colors mixed (additive) together. Light sources are “additive” color rays. We can get a white light by adding three basic colors of RGB, activating all our vision cone cells. A computer screen or a light-emitting diode (LED) source produces the color white by mixing RGB LED rays.
When we observe light, we are either observing the source of light or the reflection of the source from a surface passing (transmitted) through some media such as air, glass or water. It is possible for the transmission medium and the reflecting surface to absorb particular wavelength(s) of the light source. A flashlight is an emission source, while a wall covered with a paint containing a red dye is a reflecting surface. Also, keep in mind that light can interact with particles of matter as it passes through or reflects. All matter is made of atoms, ions and molecules with bound electrons (called standing waves), whereas light is propagating electron waves (or photon particles) traveling at or near speed of light.
Color is a psycho-physiological phenomenon that helps our brain to characterize light for vision. The colors that we see require a light source and the interaction of matter with wavelengths of light emitted from the light source. Matter is either reflecting or transmitting light, so what we see are the rays (wavelengths) of light that matter is passing through or reflecting. Light that is being transmitted (or reflected) is always the difference between the emission and absorption spectrum.
Transmitted (Reflected) Light Spectrum = Source Emission Spectrum – Absorption Spectrum
Since color is a non-scientific phenomenon based on our perception, we can model our eye receptors to determine colors. Our eyes use the three-color receptors RGB and their overlapping signals to construct a complete set of colors for our vision. These three fundamental colors can mix and produce three secondary (mixed) colors according to the following diagrams. Note that the intersection (additive mixture) of these color rays produces white, which is activation of all three cone cells simultaneously. White LED lights mix these three fundamental colors to produce “white.”
Mixed colors are products of adding the two color rays located in-between any two fundamental RGB colors producing magenta, yellow and cyan. Complementary color rays when added produce white (RGB). These are shown across from each other and connected with a dashed line. (Spoiler alert: Answers are shown in the right-hand column.)
|Medium Absorbs||Color Transmitted (or reflected)||Answers|
|R+B+G||??||Black (no color)|
|None||??||White (all colors)|
Light Sources that Emit Continuous Spectra
In this experiment, we will focus on emission sources of visible light (emissive sources). How does a substance produce light? When energy in the form of heat or electricity is added to matter, it excites particles of matter according to laws of quantum mechanics. A matter tends to emit continuous spectra when its particles are very closely bonded to one another (condensed matter) such as solids.
For example, a metal spoon heated over a flame increases the frequency of vibration of metal atoms, emitting light at the far infrared zone of the EM spectrum at lower temperatures. As the spoon gets hotter, the added energy causes atomic vibrations. Heat generally raises the vibrational energy of matter as atomic particles store the heat energy in their higher vibrational energy states. These energized states emit their stored energy as light as they keep getting energized and the cycle continues.
As the spoon gets hotter, the emitted energy includes higher energy (shorter wavelength) EM radiation and the entire spectrum shifts toward the higher energy visible light. At some point, the spoon glows red and then yellow and orange as higher energy colors are mixed and emitted. At very high temperatures, it is conceivable to expect pure white light being emitted from the spoon.
The continuous color emission from this heating process produces a blackbody or thermal emissive (TE) radiation. These spectra are often continuous and without sharp lines or emission peaks. A metal filament in a light bulb can become white hot as its temperature increases. The solar spectrum is the result of millions of temperature degrees caused by nuclear reactions taking place in the sun’s core. As sunlight travels through the atmosphere, certain wavelengths are absorbed by atmospheric gases such as H2O, CO2 and ozone, O3. The following graph shows the absolute irradiance spectra of sunlight in space, at sea level and inside a body of water. (Source: Wikipedia).
Light Sources that Emit Line Spectra
When matter is excited through heat or electricity in its gaseous state, a different kind of spectra is observed, one in which its emission appears as lines or peaks along its wavelength called the line spectrum. The line spectrum is an atomic event and is unique for each atom type of the periodic table as well as molecular substances, like unique human fingerprints. A line spectrum is caused by electrons dropping from their excited energy states to lower energy states. This electron “relaxation” process keeps repeating itself as electrons in an atom are excited with light, electricity or intense flame. Examine the emission spectrum of hydrogen as shown below and its relationship to its energy level diagram. Please note the spectra shown only includes the visible emission lines of hydrogen. Excited states of hydrogen also include ultraviolet and infrared lines that are beyond the visible range. Note that the “A” line signal of hydrogen at 410.1 nm is very weak and almost lost within the detector’s “noise” level. The “D” line for hydrogen (656.3 nm) is the strongest visible line.
Interestingly, molecules can also exhibit excitation of their bonding electrons upon heat, electricity or light. The excited bonding (or valence) electrons relax to lower energy states, giving off light energy. An ultraviolet (UV) excitation source has the power to permanently alter or break molecular bonds. For example, excessive UV exposure is dangerous to our skin and eyes and causes severe burns that may cause skin cancer. Consider sun protection when facing daylight.
In this experiment, you will have an opportunity to observe the spectra of several light sources and determine the visible light’s irradiance (in µW/cm2 units) and determine the energy and brightness (lumen) efficiency of the light source. We will use a miniature Czerny-Turner type compact spectrometer with the following operational diagram. An optical fiber cable will transfer light through a narrow opening (25 µm entrance slit) of the spectrometer. A CCD detector element (much like one in a digital camcorder) sends the wavelength/light intensity data to computer software for data analysis and graphing. To a scientist, a spectrometer and its graphical data are like having a powerful set of "super eyes" to investigate matter and its properties at the atomic scale!
The spectrometer unit of this experiment is pre-calibrated for absolute irradiance in the units of MicroWatts per square centimeter (µW/cm-2/nm-1) at each wavelength of light. A general understanding of energy and power and their units of measure is necessary. We will work with the SI units of energy and power. Energy is measured in Joules (J). Power is energy over time and is measured in Watts (W), which is Joules per second (J/s).
Power (W) = Energy (J) / Time (s)
1 W = 1 J/s
Electrical energy is commonly measured using the kilowatt/hour unit (kWh). Your home’s electrical energy usage is measured at your panel’s electric meter in kWh unit and you are charged at a certain rate in $/kWh (e.g., $0.15/kWh).
Solar to Electric Conversion (Photovoltaic or “Solar” Panels)
Our sun provides free solar energy for us daily. While, the Earth’s atmosphere absorbs a portion of the solar energy, it leaves roughly 1100 W/m2 of peak power (called insolation) at sea level on a clear day with the sun directly overhead (Source: Wikipedia).
We can generate our own electricity during the day using solar irradiance and properly designed connected solar panels. Each solar panel is rated for its power in Watt units (e.g., a 220 W panel). Each panel typically lasts for about 25 years with some reduction in its rated power. We need many connected panels (and other, auxiliary equipment) to make our own electricity. For example, installing 25 individual solar panels each rated at 220 Watts on a sun-facing rooftop, provides us with 5.5 kW of electric power (kilo=1000).
Solar panels are also rated for their solar to electric conversion efficiency. For example, a new Sanyo panel rated at 18% solar efficiency converts 18% of the incoming solar energy (called solar insolation) into electricity while directly facing the sun. Solar insolation varies based on each geographical location and season, which determines the solar intensity, sun-hours and the angle of sun’s rays.
See Problems 1 and 2 in the Pre-laboratory Assignments section.
Essentially, there are three types of sources of light: thermal emissive (TE) or blackbody, gas discharge (GD) and light emitting diodes (LEDs). Electricity energizes these light sources. LEDs are semiconductor-based lighting and currently the most efficient in producing the least amount of heat for a given light output level or brightness. Sunlight is a TE emissive light source rated at 1100 W/m2 at peak condition (no clouds) directly overhead.
Thermal Emissive (TE)
Sunlight and filament-based bulbs such as incandescent and halogen lights are TE type and work by heating a tungsten metal filament to high temperatures (1800-2700 K) using electrical current in the presence of an inert gas (and a halogen gas) within the glass bulb. The color of a halogen light depends on the filament temperature, approaching bright white as the temperature increases. These lamps lose a lot of their radiant energy in the form of heat as they give off visible light. Many halogen lamps are currently being replaced by GD and LED lights.
Gas discharge lamps work by passing a small current at a high electric potential through a gaseous element like hydrogen, producing an electric arc much like a lightning bolt through the atmosphere. The light emitted is from excited energy of electrons transitioning into lower atomic energy levels releasing most of their energy in the form of visible light. The most common example is a hydrogen lamp. GD lamps are more efficient in their visible light output and produce less heat than their TE counterparts. The light spectra from GD lamps is not continuous like the TE lamps and shows distinct electron transition peaks (or lines), thus producing a line spectra.
The most common commercial version of a GD lamp is a fluorescent light. Fluorescent lamps operate based on the line emission spectrum of mercury vapor, which produces intense high energy and ultraviolet (UV) peaks. The higher energy mercury rays excite a white (or yellow) internal phosphorescence coating, which in turn results in converting the higher energy line spectrum into a white (or yellow) light with some continuous spectrum character. A more recent development in high efficiency GD lamps is the Ceramic Metal Halide (CMH) lamp. A CMH lamp produces a bright white spectrum resembling the solar spectrum as well as line spectrum, and is used for stadium lighting as well as growing plants indoors. These lamps are generally very efficient and produce a very bright white light.
Light-emitting Diodes (LEDs)
LED lights are semiconductor based and among the most efficient light sources. They commonly come in Red, Green and Blue. The rather recent development of blue LEDs made it possible to combine these colors to create a white light. Combining an RGB set of LEDs in different intensity proportions produces white lights with different tones, also known as Kelvin (K) color temperature. Higher color temperatures (bluer) produce a whiter light. LED lamps are rapidly competing with other light sources in efficiency, cost and long life. LED lights also can be designed to mimic sunlight and can be used to grow plants indoors.
Luminous Power Efficiency
The luminous efficiency of a light source is dependent upon the total emitted visible light output divided by the electrical energy input in Watts (W) power unit:
The output light power is determined by calculating the total area under the absolute irradiance graph of the lamp’s visible spectrum (380-780 nm) multiplied by the lamp’s total illumination area. The method of calculating the area under the irradiance graph is a straightforward rectangular method calculated numerically based on the wavelength vs. power data for each light source using an Excel spreadsheet. This method is outlined at the end of this section.
The total illumination area of a light source depends on the geometry of the lamp and its light ray pattern emitted from the lamp. To keep our experimental setup and our mathematical analysis simple and relatively repeatable, we will place a common glass light diffuser over a light bulb in a simple electrical fixture. Next, we will measure the absolute irradiance at the surface of the diffuser globe. We will then use the formula for surface area of a sphere (sphere is approximate shape of the illumination surface minus the lamp base). Therefore:
The input electrical power (P) is calculated by multiplying the electrical alternating current (AC) measured in Ampere (A) unit by the input (supplied) AC electric potential E in Volt (V) unit. We will measure them both. (I: electric current)
Please keep in mind that thermal emissive (TE) sources are not as efficient because they convert a portion of their electric energy input to heat (infrared radiation). GD and LED lamps are far more efficient in their light output. A light source should emit very minimal UV (ultraviolet) radiation as this form of radiation is generally harmful to our eyes and skin. UV-A radiation is the least harmful of all UV rays in the range of 315-400 nm. UV-B is more harmful, containing higher energy. The ozone layer in the upper atmosphere absorbs most of the UV-B radiation in the 280-315 nm range. In this experiment, we will determine the UV-A, Visible and IR power of each light source using our calibrated spectrometer. Efficient light sources should emit most of their spectrum in the visible range. Typically, thermal emissive (TE) lights are the least efficient types where a significant portion of their spectra is wasted in the IR range as heat.
How to Calculate the Area Under the Absolute Irradiance Graph Using Excel
The total light power of each spectrum should be obtained for each wavelength zone according to the following ranges. Keep in mind that the lower and upper wavelength limits are determined by the spectrometer’s grating type as well as its calibration range.
Consider the following example graph showing the absolute irradiance spectrum of sunlight. You will be performing a similar Excel analysis for each light source.
You will be dividing each spectrum into tiny rectangular elements (only three out of many are shown on the above graph) covering the visible range of 380-780 nm using the irradiance data that was imported into Excel. You will use Excel to calculate the area of each rectangle and sum up all the rectangles covering the wavelength range, which is the total area under the graph. Please note that for each rectangle you will be taking the average value of irradiance between the neighboring wavelengths and multiplying that by the difference between the neighboring wavelengths. You will then use Excel’s “sum” function to add up all the “tiny” rectangular areas covering the entire wavelength range.
You are now ready to determine the total luminous power (area under the absolute irradiance plot).
Follow the instructions for Problem 3 in the Pre-laboratory Assignments section.
Perceived Light Intensity
Light sources are very commonly rated in terms of their illumination intensity as perceived by our eyes. As mentioned earlier, our sight receptors have the greatest sensitivity to green and red colors and lowest sensitivity to blue light. This illumination level is called “illuminance” and is expressed in terms of “lumens” of light per unit area (lu/m2). Numerous studies have quantified the lumen level sensitivity of the typical eye’s response as a function of wavelength (luminosity function). The eye functions somewhat differently between day (bright) and night (low light) light levels. The daytime light level response is called “photopic” (bright) and the low (night) light level response is called “scotopic” (dim) response. The photopic and scotopic luminosity curves are shown in the following plot (source: Wikipedia). Since our light sources mimic the daylight response, we will use the photopic response for our lumen calculations.
The peak of the photopic luminosity curve is at 555 nm (green). The above curve lists efficiency of the light detection at other wavelengths in comparison to green light at 555 nm. A light power of 1 watt (W) corresponds to 683 lumens (lm) of brightness (1 W = 683 lu) at 555 nm. In this context of light perception, light sources can appear brighter if their spectrum is more intense around the maximum of the luminosity response curve. The following equation (and equivalent Excel equation) can be used to approximate the efficiency at different wavelengths and convert the µW/cm2 (area) data at each wavelength to MicroLumens/cm2 (µlu/cm2).
Photopic Efficiency = 1.019*EXP(-285.4*(DH25*0.001-0.559)^2)
Microlumens (µlu) = Photopic Efficiency*DJ25*683
DH25: example cell reference where wavelength (nm) data is located.
DJ25: example cell reference where µW/cm2 data is located.
Work on these problems:
- How much does it cost to operate an AC cooling unit in the summer with a compressor motor rated at a 1570 W used for 8.75 hours? Assume that the electric rate is $0.25/kWh. How many megajoules (MJ) of energy are used? (Ans. $3.43, 49.5 MJ)
- How much money is saved annually from installation of 55 panels mounted in a location with an average of 4.0 hours per day of peak solar insolation at 1100 W/m2? Assume that each panel’s area dimension is 77 in. x 39 in. and operates at 15.5% efficiency. Assume that the average cost of electricity is $0.12/kWh. (Ans. $2,900)
- Add into an Excel spreadsheet the sample irradiance data and the area calculation column shown below. Enter the wavelength (nm) and irradiance (µW/cm2) values (shown in bold font). Don’t start from column S and row number 549. Start from a location somewhere on the top left corner of your spreadsheet and enter the indicated values. Don’t include any gaps into your spreadsheet! (Enter the value 778.880 right below 382.655). Follow the calculation logic as shown in box with the Excel example formula. Enter your own formula and copy and paste it into the entire Area column. Next, sum up the Area column values using the Excel formula “Sum(range)”.
Total area = Sum of each cell in the Area column of your spreadsheet. (Ans.= 335.81 µW/cm2)
- Ocean Optics Flame-S Spectrometer (calibrated for absolute irradiance)
- Cosine diffuser and optical fiber
- OceanView software and laptop
- Light fixture & light diffuser globe
- Different light sources (bulbs)
- AC current meter and voltmeter
Note: This experiment does not produce any chemical waste.
Install each type of light source (bulb style) by screwing it into the base of the light fixture with the light diffuser globe placed over the bulb as shown in the picture. The light measuring tip (called a cosine diffuser) will be secured in a laboratory clamp directly facing (and touching) the glass diffuser globe. Next, the absolute irradiance graph (light power vs. wavelength) will be collected from the light source when it is powered on and the spectral data will be copied into an Excel template spreadsheet. (In an educational setting, the instructor would make the Excel spreadsheet containing the raw data available to every student for further calculations and graphing.)
Diffuser Globe’s Surface Area Calculation
Glass Diffuser globe’s diameter: 14.92 cm (spherical shape)
- Calculate the surface area (m2) of the spherical globe. This is the total light illumination surface area.
Data Summary Table
|Power, (Vis), W|
- Which light source produced the highest amount of visible light using the least amount of electricity? Which values on your data summary table support your answer? List these values with correct units.
Light source: _________________ Value: ____________
- For the following data calculate the total power (W) and total lumens (lu) considering the listed photopic light efficiencies. Assume a total light collection area of 1.75 m2. Show all your calculations. Box your answers.
3. A 55 W (electric power) light source was measured for its illuminance and 450 lu were reported.
- Calculate the luminous power efficiency.
- If the absolute irradiance measured at 655 nm for this light source is 1.75 uW/cm2, how many photons per second (per cm2) are reaching the detector at 655 nm?
4. The following plot (generated from OceanView spectroscopy software) shows the spectra of two different light filtering sunglasses (brand A and B) filtering daylight including the unfiltered blue-sky spectrum. Analyze these spectra. What are the similarities and differences between these sunglasses? How do their spectra compare with the blue-sky spectrum? Explain.