Poor Man’s Spectrometer: Estimating Lamp Spectral Quality for Less Than $30


An inexpensive spectrometer from the
Astronomical Society of the Pacific.

It is little secret that the spectral quality of various lamps
has been a debating point among us in the reef-keeping hobby.
Quantification of lamp output quality is, at best, an expensive
proposition. However, qualification of spectra is so inexpensive
that even the budget-conscious hobbyist can afford a
‘spectrometer’ (these are marketed under the name
‘spectrometer’ and since they have a graduated scale,
the term is technically correct). There are several
‘instruments’ available for just a few dollars from
various suppliers. Even better, they are extremely simple to use
and can provide valuable information to hobbyists.

These devices will allow quick estimates of light quality. The
possibilities are limited only by your imagination. We’ll
discuss a few of these at the close of this article.


I have two ‘spectrometers’ from two sources
– my favorite is a unit purchased from the Astronomical
Society of the Pacific for a paltry $29.95 (plus shipping, taxes,
etc.). It has features that the less expensive ($6.95)
spectrometer from Edmund Scientific does not. Both are made of
plastic and house a diffraction grating for spectral separations.
Each is marked with a scale. That is pretty much where
similarities end.

Available Instruments

Reef hobbyists usually have collected a substantial amount of
equipment after being in the hobby for a few years. Some devices
have found their way to the ‘bone yard’ while others
are of great use. Although the spectrometers discussed in this
article are inexpensive, I can save anyone a little time,
aggravation and perhaps a few bucks if you’re in the market
for an item such as this. The suggested suppliers are of course
not the only sources, but I have had dealings with both and have
found them to provide prompt reliable services.

Spectrum Card

Spectrum Card, Night Spectra Quest. A small diffraction
grating is sandwiched between two credit card-size pieces of
cardboard and allows a glimpse of spectral characteristics.

Edmund Scientific
offers this device as part #3053066 for $4.95. A small
instruction booklet is included and its contents seem geared
toward grade-schoolers. Five dollars isn’t much money at
all, but I felt annoyed that I had spent this much plus shipping
on something as useless as this. I do not recommend it for
‘aquaria’ purposes.

Classroom Spectrometer

This item is also available from Edmund Scientific and is far
superior to their Spectrum Card (see above). The spectrometer
body is made from sturdy plastic and uses a diffraction grating
to split light into its various spectral components. The spectrum
falls upon a graduated scale (400-700nm only), which is viewed
through a small aperture. The unit’s compactness is also
its greatest drawback. The scale is not adjustable as those of a
pricier model, and the small display is sometimes difficult to
read. Edmund advertises an accuracy of ~50nm. This is fine for
demonstrating the concepts of spectra and diffraction gratings
and is a value at only $6.95 (plus shipping and other nominal
costs). If your funds are extremely limited, and you’re
looking for something to ‘guesstimate’ spectral
quality then this item is for you. Stock number is 3082305.


Project STAR Spectrometer

The Spectrometer from Project Star is my personal preference
of all the spectrometers in its class, and it is also the most
expensive at $29.95 plus shipping (~4X the price of its
competitor – the ‘Classroom spectrometer’ –
above). Its superior features make it worthy of consideration. It
is relatively large and this allows for easy handling and
viewing. The internal scale is easy to read and features scales
for wavelength (in nanometers) and energy (in electron volts).
The scale is engineered to allow for calibration against the
mercury discharge (546nm) of almost any fluorescent lamp –
it is easy to do (this is a real plus if the instrument gets
dropped or bounced around). The calibration feature is not
available on the other spectrometers I have seen and tested. The
plastic body is not particularly robust and could be crushed
fairly easily. The sections of the housing are held together with
brass brads, so it will not easily pop apart.

A lot engineering went into this inexpensive, handheld device
(mine says it was developed by the Harvard-Smithsonian Center for
Astrophysics). It is available from the Astronomical Society of
the Pacific
and follow the link to the ‘AstroShop’). The part
number is OA160. Edmund also offers this spectrometer, but at a
higher price of $34.95 plus nominal fees.

The graduated scale is from 350nm to 750nm, but it will
not allow a visual recognition of near-infrared or
ultraviolet energy – it will allow you to see only those
wavelengths which your eyes normally perceive. However, some
persons have an ability to ‘see’ into the UV range
(my aging eyes can see a bandwidth of about 410nm (violet) to
‘red’ at ~660nm).

As with all equipment – read and follow safety
precautions – do not look directly at the sun, any source
of ultraviolet energy or extremely bright light source! Instead
use a reflective white surface and ‘bounce’ the light
into the spec.

I highly recommend this device if you’re looking for an
easy and inexpensive way to estimate spectral qualities.

Keep Records!

I have an opinion and I’ll state it here – good
record-keeping should always be practiced! This starts when lamps
are replaced (mark your calendar when replacing metal halide
lamps, while actually marking fluorescent lamps’
replacement date with an indelible marker). It’s not a bad
idea to photograph spectra of new lamps and then every six months
(or more often if your schedule allows) in order to estimate
spectral degradation. I prefer the use of a digital camera since
older film technology leaves too much leeway with the


Photographing spectral displays of the Project STAR
spectrometer is very easy, provided your camera has a macro mode.
The photos accompanying this article were taken with an Olympus
C-5050 (5-megapixel) in the macro mode. Post processing cropped
the photographs and contrast plus brightness were moderately
adjusted – but these are representative of this
spectrometer’s display (see Figures 1-3).


Figure 1. Spectral signature of a CoralVue
250-watt 20,000K metal halide lamp. Overall, the lamp appears
crisp blue- white (judged by the scales’ coloration).


Figure 2. Spectral signature of an Iwasaki
400-watt 6,500K metal halide lamp – a true full spectrum
lamp. Compare its spectral quality to that of sunlight (Figures
4 and 5).


Figure 3. Spectral signature of an
inexpensive fluorescent desk lamp. Compare the scale’s
yellow-orange tint to the ‘whiteness’ of those in
Figures 2 and 3. Figure 6 identifies line spectra of various
fluorescent compounds – can you determine which this lamp


Interpreting the Results

Lamps used for aquaria lighting will lose output over a period
of time and spectral qualities will shift. This degraded
performance could be due to various factors, including
‘sputtering’ of electrodes (coating the inside of the
tube with metal), loss of halides, etc. Figure 4 shows a few of
the spectral spikes and the responsible elements and compounds
commonly seen in metal halide lamps. Figure 5 does the same for
fluorescent lamps.


Figure 4. Emission spectra of various
compounds often found in metal halide lamps.The spectrum is
that of natural sunlight.


Figure 5. Elements often found within
various fluorescent lamps.

Potential Uses – Use Your Imagination!

The Project Star spectrometer will enable one to examine many
different parameters. The obvious determination is that of
initial lamp spectral quality, followed by degradation of
spectral output over a period of time. What effects will that new
skimmer or activated carbon have on transmitted light quality
– especially in deeper aquaria (use a white reflective
surface placed within the aquarium at a 45ºangle). What is
the peak fluorescence of that nice Acropora you just
brought? Will that fluorescent color shift under your lighting
system – If so, to what wavelength? Simple observations of lamp
spectral quality and shifts in coral fluorescence could help
unlock some of the remaining mysteries of anthozoan pigmentation.
That’s the beauty of these inexpensive devices (especially
the spectrometer from Project Star) – they can deliver
quality information at prices practically unknown in the
reef-keeping hobby.


  1. Kaufman, J. (ed.), 1984. IES Lighting Handbook.
    Illuminating Engineering Society of North America. New
  Advanced Aquarist, Advanced Aquarist
Dana Riddle

 Dana Riddle

  (120 articles)

I have been an aquarist since 1964 and a reef hobbyist since the mid-1980’s. I am the owner of a small laboratory (Riddle Aquatic Laboratories) that specializes in investigation of interactions between light and water motion & photosynthetic organisms (especially corals). The results of this research, resulting in almost 250 articles, have been published in Advanced Aquarist Online, Aquarium Frontiers, Koralle, Freshwater and Marine Aquarium, The Breeders’ Registry, Aquarium Fish, Marine Fish Monthly and others. My first article was published in a 1984 SeaScope and relayed my experiences with a refugium – an idea that would catch fire about a decade later. I have had the honor of making over 60 presentations to various groups, including national conferences such as the Marine Aquarium Conference of North America (MACNA) International Marine Aquarium Conference (IMAC), PetsFestival (Italy), regional conferences, and local clubs. I received the Marine Aquarium Society of North America (MASNA) Aquarist of the Year Award in 2011 at the MACNA conference in Des Moines.

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