Color and Rate of Heat Absorption

 Shunsuke Kariya: 2006

Introduction: background Information

When light is incident on a surface it can be reflected, absorbed, or scattered. Smooth surfaces reflect and rough surfaces cause diffuse scattering. A surface that diffusely reflects all wavelengths equally is perceived as white, while a surface that absorbs all wavelengths equally is perceived as black. Reflection can be diffuse or specular (like a mirror). A mirror reflects all wavelengths equally, but is not perceived as white because it is smooth. Similarly, a black object can reflect (be shiny) because of a smooth finish. When a black object and a white object are exposed to sunlight, the black absorbs sunlight and gets hot while the white reflects sunlight and remains cooler. The purpose of this experiment is to examine how a change in color affects the rate of heat absorption.
 
Research Question
 
What is the relationship between color and the rate of heat absorption?
 
Hypothesis
 
The darker colors (black, blue, green etc) are expected to absorb more heat and to rise in temperature faster a than the lighter colors (white and yellow)
 
Explanation
 
Since the lamp has an approximate black body emission curve much of the energy will be at the red to infrared end of the spectrum. Cans that reflect at the blue end will therefore be absorbing more energy per second and will rise in temperature faster.
 
Materials and Procedure

Seven identical thin walled metal cans are lapped with 7 different colored papers of the same thickness and weight per square cm. The colors were white, red, orange, yellow, green, blue, and black, (figure 1). An infrared temperature sensor (heat gun) was prepared. This device measures the surface temperature of object by detecting infrared emissions.

Part 1

Calibration
 
To make sure the temperature measurement is not affected by the color of the surface the cans were filled with the same amount of tap water and left for 5 minuets. This equalized the surface temperatures of all cans. The temperature of water was measured with a thermometer.

The surface temperatures of each of the seven cans were measured with the heat gun. The water in the cans was then heated to 40°C and the surface temperatures were again measured with the heat gun.

Fig 4 - the laser spots (three in all) show the area over which the temperature measurement is being taken.

Table 1

The temperature measurements are the same within ± 0.4°C for at 26° and to within ±0.5°C at 40°. The variations appear to be random and not color dependent. There is a slight increase in uncertainty when temperature gets higher but the increase in uncertainty is considered to be negligible for this experiment. The color of the paper has no significant effect on the functioning of the heat gun.

Part 2

Measurements

A 500 Watt quartz halogen lamp was prepared as a heat source. The light and can were set apart by 30 cm. The can was exposed to the light for 5 minuets and the surface temperature was measured every 20 seconds. During the temperature measurements the central laser spot was centered on the can.

Table 2 lists the temperatures of the different colored cans on exposure to the light.

Data Analysis

The data from table 2 is plotted in Graph 2. The graph is the plotted data points of heat absorption of 7 colors. The error bars of ±0.5°C are the size of the point protectors and have therefore not been shown. To find the rate of heat absorption for each color, liner equations are fitted to each set of data points. The slope of each line is listed in Table 3.

Table 3

Discussion

The initial increase in temperature after 20 seconds is color dependent. It appears that the colors reflect progressively more energy in order, from white - red - (yellow/orange/green) - blue, and then black. White is the best reflector and black is the best absorber as expected.

As time passes the temperature of each surface continues to rise but the colors (including white) all follow very similar lines at a lower rate of increase than the black. At five minutes, the order of temperatures remains the same as it was at 20 seconds, but equilibrium has not yet been reached.

Remarkably the slow rate of increase in temperature from 20 seconds to five minutes is very nearly the same (within errors) for all the colors including white. A similar result has been reported by Fantozzi et. al. in a study of the heating effect of focused sunlight on three colored papers. The expectations, with regard to the properties of the black and white papers have been confirmed but it is surprising that the colors have such similar temperature-time curves from 20-300 seconds.

Evaluation

There were many precautions taken to ensure that the data was reliable.

Papers of the same weight and thickness in a good range of colors were used. The papers were wrapped tightly on identical thin-walled aluminum cans. The room temperature was stabilized with air conditioners. The papers were exposed to the same light at the same distance and orientation for the same length of time. The heat gun was calibrated to establish its accuracy (±1°C). In spite of these precautions several problems remained.

1 Air conditioners caused some random movement of cold air may have affected the surface temperature of the cans.

2 The heat gun was hand held. Measuring the temperature of the cans over a slightly different area may also have contributed to the random errors.

3 Thirdly, to properly interpret the data, it would be desirable to have absorption spectra for the pigments and an emission spectrum of the quartz-halogen lamp.

Errors due to the first points could be reduced by turning off the air conditioners well before measurements were taken and allowing the room to come to a steady equilibrium temperature. and by fixing the heat gun on a clamp stand.

The third point is more difficult to overcome. Perhaps in the future the spectra could be measured or pigments could be used for which the spectra are known.

Suggestions for further work

Wrapping the papers around thin walled aluminum cans allowed the conditions to be well controlled but the presence of the can may have altered the temperature-time curves because of conduction from the paper. It would be interesting to compare the temperature-time curves for unsupported paper and for paper wrapped around a can.

It would also be interesting to compare in detail the temperature-time curves for the different papers over the first 20 seconds. The display panel on the heat gun could be filmed to make the data collection process more reliable over a short time.