CH332 Lab 5: Color of Art
By this point in your Colby Chemistry career most of you have used spectrophotometers to
measure absorbance or transmittance spectra for both quantitative and qualitative analysis. In this lab
exercise you will use the Ocean Optics spectrometer to measure the relative irradiance of lights in the
Colby Museum of Art.
Consider the two pictures below. The first picture has a normal color balance expected for an
outdoor photograph taken in direct sunlight (5600 K). The second image has been adjusted to have 20%
more blue and 20% less red emission intensity. This is designed to simulate the effects of placing the
picture under fluorescent lights.
Read more about colour theory on the following very interesting linked pages: Colour Theory, Color and
Vision, Color Vision (How to Choose Studio Lighting).
Normal color balance (outdoor light)
Adjusted results for simulation of fluorescent light
The figure below is the emission spectra of a tungsten-halogen lamp operating at 3100K. You may
calculate the emission spectra for any black body radiator using Plank's Law for spectral radiant exitance
from a non-perturbing aperture in a black body. (See Laws of Radiation)
The figure below is the emission spectra of a tungsten-halogen lamp operating at 3100K. You
may calculate the emission spectra for any black body radiator using Plank's Law for spectral radiant
exitance from a non-perturbing aperture in a black body. (See Laws of Radiation)
The Excel spreadsheet included with this handout on the CH332 lab webpage will assist with these
calculations.
We will measure the relative irradiance of several light sources in the Museum of Art. Irradiance
is the amount of energy at each wavelength emitted from a radiant sample. Absolute irradiance is the
measure of light in absolute terms. Relative irradiance is a comparison of the fraction of energy the sample
emits and the energy the sampling system collects from a lamp with a blackbody energy distribution
(normalized to 1 at the energy maximum). Relative Irradiance is calculated as follows:
where RI is the relative irradiance, S is the signal, R is the reference spectra (typically a W-halogen source
at 3100K), D is the instrument dark and Rc is relative energy of the reference spectra calculated from the
user defined color temperature. At first it seems odd to scale the signal by the reference signal,
Rc. However, Rc is the relative energy so all the output is scaled by photon energy, a direct measure of the
light available to your eye.
The attached spreadsheet allows you to calculate Rc for any black body (Black Body Emission
Spectra.xls) .
Of course, what your eye really senses is modified by the response function of the human
eye. Once we have the absolute intensity of each light source we can multiply these values by the human
eye weighted Response Function to get perceived color spectrum. The eyes senses light intensity with two
types of receptors, rods and cones. The rods are much mores sensitive, but have little color
sensitivity. The cones have three different pigment types that absorb light in the red, blue, or green. The
weighted response function shown below is computed by summing the weighted spectral response of the
red, blue, and green cone pigments as summarized in the manuscript by Dartnall et. al, 1983 (found in the
CH332 Information binder in the lab).
*taken from Dartnall et. al, 1983
Procedure:
The manual for our OOI USB4000 instruments is attached. (Spectra Suite Manual.) The specific
instructions for irradiance are included in the manual.
1.
In the laboratory calibrate the USB4000 for relative irradiance using a W-halogen source
(3100 K).
2.
Disconnect the source and connect the light focusing optic (photodiode with focusing optics, and
fiber optic connection).
3.
Measure the emission spectra of the fluorescent light in the PQ (K205) lab. Using the save icon in
the Spectra Suite window, save the spectrum (graph) to the desktop and the output data (use the
save to clipboard icon in the window with tab delimited file form and save directly into excel).
4.
Move the spectrometer, computer, and optical stand to the art museum.
5.
Acquire the emission spectra of six different illumination sources (background, full sun, filtered
sun, and the three different lights attached to the light box: incandescent, fluorescent and
LED). Photograph your art sources under each type of illumination for documentation. (We will
supply the camera). Save spectrum to the desktop and the data output in excel.
6.
The output files will be relative emission energy (compared to our 3100K source) as a function of
wavelength. Changing the distance from the source to the detector will change the measured
intensity, but not the shape (what wavelengths are prominent) of the emission curve. We are more
interested in the shape of the light intensity since this influences our perception of color.
7.
Rename and save the output data file as an excel file. (Rename each sheet to organize data?) Email
the file to yourself.
8.
Look at the background data. Make each negative value = 0 (use the IF function in excel). Save.
9.
Subtract the background from each illumination data file.
10. Scale each curve by the maximum value of each data set (0-100%). Plot and compare all six data
files. (Use the MAX function in excel. Use (use a colon to define the column…not a comma!!).
11. Now scale each of the files by the relative response function of the human eye. Plot and compare
all six scaled files. (The LOOKUP function in Excel will help with data organization and
evaluation.)
12. Why would you use the relative emission spectra instead of the human eye spectra to evaluate
“pigment bleaching” rates in paintings?
13. Using a photo editor, simulate the effect of the three different illumination methods on the "color"
of one of the paintings from the museum. Refer to the links in the introductory section for more
details on the human perception of color.
To be handed in: You will work as a group to prepare a report using ACS format to report and explain
your irradiance data and the effect of the different kinds of light on the perception of art.