Calibrating Your Digital Camera
1. Set up a piece of white paper so that it is
uniformly illuminated by indirect sunlight. Choose a place where the light is
bright, but you should not see any shadows on the paper. The most important
thing is that the paper is illuminated evenly, with no bright spots or shadows.
Choose a time and place during which it is unlikely there will be any changes in
2. Position your digital camera so that the white
paper fills the entire field of view. The precise distance from the camera to
the paper is not critical.
3. Put the camera in manual mode and make the
a. Manually adjust the focus so
that the camera is focused on the white piece of paper. Once you set the focus,
do not change it during the calibration.
b. Set the camera’s sensitivity to
c. Set the aperture to f/2.8.
d. Set the image resolution to a
e. If your camera has a self-timer
feature, set it so that the camera shutter opens a few seconds after you press
the button to take a picture. This minimizes shake.
4. Now that your camera is focused on the white
piece of paper, with the settings adjusted correctly, take a series of photos
at different shutter speeds, varying by a factor of 2 each time. (I used:30,
15, 8, 4, 2, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, and
1/1,000 sec shutter speeds.)
5. Repeat step 4. This will give you two complete
series of calibration photos. You will compare the two sets of photos later in
6. Download all of the calibration photos onto your
7. Make two data tables in your lab notebook to keep
track of the pixel value information for each calibration photo. One data table
will be for the first set of calibration photos. The other data table will be
for the second set of calibration photos.
8. Measure the average pixel intensity of each photo
a. Start ImageJ.
b. Open the first calibration photo
using the “File/Open…” menu command.
c. Click on “Analyze” and
select “Histogram” from the drop-down menu.
d. A histogram of the pixel values
in the photo will open in its own window. You will use this histogram to
measure the average pixel gray value in each image.
e. Record the Mean, StDev, Min,
Max, and Mode in your data tables. StDev is short for “standard
deviation”. Min and Max are short for minimum and maximum, respectively.
Mean is another name for the average. The mean of this histogram is the average
f. Click on “File” and
select “Open Next” to open the next image file.
Repeat steps 3.d.–3.f. until you
have analyzed all images in both sets of calibration photos.
9. Make a calibration curve by graphing the average
pixel value (the mean of the histogram) on the x-axis and exposure time (in
seconds) on the y-axis. Use a logarithmic scale for the x-axis and a normal
(linear) scale for the y-axis. Your graph will have two data series, one for
each set of calibration photos. Choose different colors or symbols for each
a. This is a “semi-log”
10. Look at your graph and compare the calibration
curves from each set of photos. The two curves should overlap or only be
slightly separated. If there is a lot of space between the two calibration
curves or if one of the curves has a very different shape from the other, you
will need to repeat steps 1–9, making sure that the lighting conditions are the
same for both sets of photos.
Taking Skyglow Photos
Now that you have finished calibration, you are ready
to measure skyglow. Pick three or four places where you would like to measure
skyglow. Choose places you think will have different amounts of skyglow.
a. Make sure it is around the same
time of night and that there are no clouds.
2. Travel to your first location with all of your
supplies. Pick the one you think will have the most skyglow. In your lab
notebook, write down the address of your first location. Include a brief
3. Set up your camera to take skyglow photos. Make
sure the camera is in full manual mode. It is necessary to use the same camera
settings at each site you visit. These are the same settings you used for
a. Set the camera’s sensitivity to
b. Set the aperture to f/2.8.
c. Set the image resolution to a
d. If your camera has a self-timer
feature, set it so that the camera shutter opens a few seconds after you press
the button to take a picture. This minimizes shake.
3. Lay your towel or rag on the ground, then lay the
camera down on it, with the lens pointing toward the sky. If you have a tripod,
you can mount the camera on the tripod and then point the camera toward the
4. Double-check that your camera’s field of view
does not include the Moon, street lamps, or house lights.
5. Take skyglow photos.
6. Repeat steps 2–5 for each of the remaining
locations where you plan to measure skyglow.
Using Your Calibration to Measure Skyglow
1. Use the ImageJ software to measure the average
pixel value in each skyglow image by following the procedure in step 8 of the
Calibrate Your Digital Camera section. Make a data table in your lab notebook,
and record the mean, standard deviation, minimum, maximum, and mode of each
pixel value histogram.
2. Because all of your skyglow images were taken
with the same camera settings and exposure time, you can use the calibration
curve to determine an “equivalent exposure time” (EET) for each
skyglow photo. The EET is how long the exposure time would have to have been
under calibration conditions to reach the same average pixel value as measured
in the skyglow photo.
3. Convert the average pixel value in each skyglow
image to an EET. Record the EET for each image in your lab notebook.
4. By converting the average pixel values of each
skyglow image into an EET, you can determine how much brighter or darker one
location is compared to another.
5. Determine which of your skyglow locations had the
smallest EET. This is the location with the darkest skyglow.
Review of the Literature
For most of Earth’s history, our universe
of stars and galaxies has been visible in the darkness of the night sky. From
our earliest beginnings, the display arrayed across the dark sky has inspired
questions about our universe and our relation to it. The history of scientific
discovery, art, literature, astronomy, navigation, exploration, philosophy, and
even human curiosity itself would be diminished without our view of the stars.
But today, the increasing number of people living on earth and the
corresponding increase in inappropriate and unshielded outdoor lighting has
resulted in light pollution—a brightening night sky that has obliterated the
stars for much of the world’s population. Most people must travel far from
home, away from the glow of artificial lighting, to experience the
awe-inspiring expanse of the Milky Way as our ancestors once knew it.
Light pollution is light that is
not being efficiently or completely utilized and is often pointed outwards or
upwards and not downwards. Also known as skyglow, light pollution occurs from
both natural and human-made sources. The natural component of sky glow has five
sources: sunlight reflected off the moon and earth, faint air glow in the upper
atmosphere, which results in a permanent low-grade aurora, sunlight reflected
off interplanetary dust(also known as zodiacal light), starlight scattered in
the atmosphere, and background light from faint, unresolved stars and nebulae,
which are celestial objects or diffused masses of interstellar dust and gas
that appear as hazy smudges of light. Natural sky glow is well quantified.
However, in the discussion of sky glow it is mainly human-made sources that are
lighting also increases night sky brightness and is the human-made source of
sky glow. Light that is either emitted directly upward by luminaires or
reflected from the ground is scattered by dust and gas molecules in the
atmosphere, producing a luminous background. It has the effect of reducing
one’s ability to view the stars. Sky glow is highly variable depending on the
immediate weather conditions, the quantity of dust and gas in the atmosphere,
the amount of light directed skyward, and the direction from which it is
viewed. In poor weather conditions, more particles are present in the
atmosphere to scatter the upward-bound light, so sky glow becomes a very
visible effect of wasted light and wasted energy.
glow, while affecting almost everyone, is of most concern to astronomers since
it reduces their ability to view celestial objects. Sky glow increases the
brightness of the dark areas of the sky, which reduces the contrast of stars or
other celestial objects against the dark sky background. Astronomers typically
like very dry clear dark nights for observing. A typical suburban sky is 5 to
10 times brighter at the zenith than the natural sky. The zenith being the
angle that points directly upward, or 180°, from the observation point. In city
centers, the zenith may be 25 or 50 times brighter than the natural background.
are three types of skyglow, each falling on the skyglow spectrum. Technically
speaking, three main types of light pollution include glare, light trespass and
skyglow (in addition to over-illumination and clutter). Glare, from unshielded
lighting is a public-health hazard—especially the older you become. Glare light
scattering in the eye causes loss of contrast, sometimes blinds you temporarily
and leads to unsafe driving conditions, for instance. Light trespass occurs
when unwanted light enters one’s property, for example, by shining unwanted
light into a bedroom window of a person trying to sleep.
pollution doesn’t just affect astronomers.
The negative effects of the loss of the night sky might seem intangible.
But a growing body of evidence links the brightening night sky directly to
measurable negative impacts on human health and immune function, on adverse
behavioral changes in insect and animal populations, and on a decrease of both
ambient quality and safety in our nighttime environment. Astronomers were among
the first to record the negative impacts of wasted lighting on scientific
research, but for all of us, the adverse economic and environmental impacts of
wasted energy are apparent in everything from the monthly electric bill to
are ways of measuring sky glow. This is no easy task, because many factors play
a role in sky glow. One must not only consider the lighting, but also the angular
distribution of the light emitted from the luminaire, the light reflected from
the ground and its angular distribution, as well as atmospheric effects of
humidity and the interaction of light with aerosols (particles in the
atmosphere that may be caused by manufactured pollutants, fire, volcanic
eruptions, etc.), all of which can change from moment to moment.
are many different ways to help improve the situation. Aside from houses,
street lamps are one of the biggest causes of light pollution. The most
polluting are the lamps with a strong blue emission, like Metal Halide and
white LEDs. Change from the now widely used sodium lamps to white lamps (MH and
LEDs) could produce an increase of pollution in the scotopic and melatonin
suppression bands of more than five times the present levels, supposing the
same photopic installed flux. This increase will worsen known and possible
unknown effects of light pollution on human health, environment and on visual
perception of the Universe by humans. There is a measurable criterion to
evaluate the lamps based on their spectral emissions and scientists suggest
regulatory limits for future lighting.
is an important topic to bring attention to. There are epidemiological
evidences of increased breast and colon cancer risk in shift workers from light
pollution . An inhibition of the pineal gland function with exposure to the
constant light (LL) regimen promoted carcinogenesis whereas the light
deprivation inhibits the carcinogenesis. Treatment with pineal indole hormone
melatonin inhibits carcinogenesis in pinealectomized rats or animals kept at
the standard light/dark regimen (LD) or at the LL regimen. These observations
might lead to use melatonin for cancer prevention in groups of humans at risk
of light pollution.
are many different ways to help lessen the amount of light pollution. The most
important thing to do is to reduce light escaping your home and to direct the
illumination down, not up. These aren’t the hardest things to do and can even
save you money. Using dimmer light switches also can help. Light pollution is a
serious issue; it is widespread and has many bad side effects. Additionally it doesn’t just harm scientists;
it helps to cause many diseases and conditions. It also affects the sleep
pattern of nocturnal animals.
choose to do this topic since I have strong interests in photography and
astronomy. I also became more interested in it when I read up on how serious of
an issue light pollution is. I believe that it is a serious issue for everyone.
If all directional technology suddenly (i.e Gps, google maps,etc.) failed, we
wouldn’t be able to see the night sky to navigate. In my experiment I will be
photographing different areas’ night skies on multiple nights. I will then run
the through ImageJ and compare and contrast which have more or less skyglow. My
hypothesis is that the area with more street lights will defiantly have more
skyglow than the rest, but the high up areas mainly Vista Point will have less.