The Journey of a Photon –
from Spark to Spectra
Bring the Journey of a Photon
to Your Classroom or Lab!
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JOURNEY of a
AN EXCITING START
The photons used in spectroscopy encounter many components and
undergo a variety of processes before registering as a spectrum on the
screen. What exactly happens to these photons once they enter an
Ocean Optics spectrometer? Let’s look at their journey in layman’s terms.
1
• The photons used in spectroscopy
PHOTON:
from Spark to Spectra
encounter many components and undergo
various changes before registering as
spectra in software. What happens to these
photons once they enter an Ocean Optics
spectrometer? The journey starts here.
• Photons are created through reactions in
the sun or stars, or emitted from lamp
bulbs, LEDs or lasers. Each photon is
created with a specific wavelength, which
it will carry throughout its lifetime.
A BUMPY ROAD
TO THE SLIT
2
• As photons travel through space,they may
be reflected, transmitted or absorbed by
4
5
materials they encounter.
3
• Different materials will reflect, transmit
or absorb photons of different wavelengths,
so each interaction will filter, redirect or
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eliminate photons of various wavelengths.
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DIFFRACTION
• The collimating mirror reflects
photons to the diffraction
grating, which splits the photons
by wavelength.
• This is the most important
step in separating the
6
COLLIMATION
SINGLE FILE PLEASE
• Photons entering through the slit are
• The slit is a very narrow aperture
diverging in space. A collimating
through which a stream of well-
mirror “straightens” their path for top
efficiency.
• Collimation ensures all photons are
directed photons traveling in a
consistent direction can flow.
• The wider the slit, the more photons
A GUIDED PATH
• Fiber optic cables route light from one
point to another, without interference
from ambient light.
• The light is guided to the spectrometer
Photons in Action
via the SMA 905 connector. Its
traveling parallel to each other, and
can get through, but at the expense of
bulkhead provides a precise locus for
collected light by wavelength,
won’t spread or scatter in unwanted
reduced optical resolution.
the optical fiber and slit.
so that each wavelength can
directions.
Learn how people are connected by light – measuring it, moving it, applying it – in pursuit of research,
education and knowledge. Visit oceanoptics.com/
application.
be measured discretely.
7
8
Spectroscopy
on the Move
• Detecting water on the
moon
• Determining soil and
water quality
• Measuring ozone levels
on Mt. Everest
• Monitoring crop health
from drones and airplanes
Can only
“fill” so far...
Photons
collects photons of a specific
Thanks to advances in detectors, microprocessors and fiber optics, spectroscopy is no
longer confined to the lab.
• Authenticating fuel
supplies at checkpoints
9
CHARGE!
• Each pixel acts as a well that
wavelength range.
• At the start of each integration
period, the well is “full” of
FOCUS CAREFULLY
• The diffraction grating spreads light
across the focusing mirror, which then
THERE IS ALWAYS A
TRADE-OFF
• Smaller range = higher resolution
• Larger range = lower resolution
voltaic charge.
• The longer the integration time,
can be collected.
each pixel collects photons from a
“Somewhere, something
incredible is waiting
to be known.”
very narrow range of wavelengths.
– Carl Sagan
pixels – a 1-dimensional line where
A/D converter
changes voltage
into counts;
the photon is
now data
fully depleted, that pixel is
“saturated” and no new signal
the detector.
• The detector is a linear array of CCD
Stored electrons
analog signal
the more photons can be
collected. Once the charge is
focuses light at each wavelength onto
• As each photon strikes the
detector, its energy is released;
we will not see it again.
10
ENTER THE
MATRIX
• At the end of an integration, the charge
level is read from all the pixels on the
detector.
• This read-out (still in analog volts) is
passed into an ADC (Analog-to-Digital
Converter), which converts each pixel’s
voltage into “counts.”
• This is when the photon becomes data.
As one journey comes to an end in the
spectrometer, a transformation to
electronics and software takes us on a
new path – and a story for another time.
Original
3 bit
1 bit
4 bit
2 bit
5 bit
ONE JOURNEY ENDS,
ANOTHER BEGINS
This concludes our photon’s journey.
From its creation, to its travels within the
spectrometer, to its arrival at the
detector pixel cell, the photon navigates
an elegant path. As each photon’s
“fate” is recorded and digitized into
data, additional stops along the way
lead us to graphable spectra in a format
1. An Exciting Start
we can understand.
For more information about our spectrometers, visit www.oceanoptics.com or call one of our Application Specialists:
US - +1 727.733.2447 • Asia - +86 21.62956600 • Europe - +31 26.3190500
The photons used in spectroscopy may be created through reactions
in the sun or stars, or emitted locally from lamp bulbs, LEDs or lasers.
Photons can even be excited from everyday materials as fluorescence
or Stokes scattering (Raman). Whatever its origin, each photon is
created with a specific wavelength, which it will carry throughout its
lifetime (whether nanoseconds or billions of years).
2. A Bumpy Road to the Slit
As photons travel through space, they may be reflected, transmitted,
scattered or absorbed by the materials they encounter. By looking at
the resulting light as compared to the initial amount of light, it is possible to learn about the properties of the material or sample encountered. This is because the probability of a photon interacting with a
material in a certain way varies with that material’s chemical and
physical structure, and also with the wavelength of the photon itself.
Each interaction with a material will filter, redirect or simply eliminate
photons of various wavelengths.
3. A Guided Path
Fiber optic cables are a convenient
method to safely route light from
one point to another. Multimode
fibers guide light through total
internal reflection, acting as a light
pipe to redirect light from one
location to another without interference from ambient light. A fiber
can even “bend” light around corners, and greatly simplifies the process of routing light to a spectrometer.
450 nm) are reflected at one angle, while “red”
photons (around 700 nm) are reflected at a larger angle.
This is the most important step in separating the collected light by wavelength, allowing each wavelength to be
measured discretely.
Most Ocean Optics fibers use an SMA 905 connector at
their tip to connect the spectrometer, providing a snug,
alignment-free coupling between the fiber’s end and the
entrance point into the spectrometer: the slit.
4. Single File, Please!
The slit is a very narrow aperture through which a stream
of well-directed photons traveling in a consistent direction
can flow. A typical slit is only 1 mm tall, and anywhere from
5 µm to 200 µm wide. Most slits are
rectangular, but some may be oval for
improved optical performance. The
wider the slit, the more photons can
get through (higher throughput), but
at the cost of reduced optical resolution (higher FWHM, which means
some peaks may appear wider than
they truly are).
5. Collimation
The photons entering through the slit are still diverging in
space (the beam expands after passing through the slit), so
their first port of call is a collimating mirror. This has a
focusing effect to help the photons travel parallel to one
another, so that they won’t spread or scatter in unwanted
directions.
6. Diffraction
The collimating mirror simultaneously reflects the
photons to a reflective diffraction grating, which splits
photons out by wavelength. “Blue” photons (around
7. Focus Carefully Now
After leaving the diffraction grating, the newly
dispersed light is sent to a focusing mirror, which
reflects light of each wavelength onto the detector
while focusing it slightly. The detector is a linear array of
CCD pixels — basically a digital camera, but in a
1-dimensional line instead of a 2D rectangle. Each pixel
collects photons from a very narrow range of wavelengths (anywhere from 15 nm to 0.02 nm, depending
on the spectrometer’s configuration).
8. Sacrifices Must be Made
While a variety of spectrometer slit and grating combinations are possible to achieve the needed range and
resolution, there are compromises that must be faced.
A smaller range generally allows higher resolution for
applications such as laser characterization, while a
larger range can often mean lower resolution, which is
acceptable for more general chemical studies like
protein absorbance. The two can be balanced well,
however, with the use of a larger optical bench, as in
the HR2000+CG, which covers 200-1050 nm with
~1.5 nm (FWHM) optical resolution.
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9. Charge!
Each pixel of the detector acts as a well that collects
photons of a specific wavelength range. The well starts
each integration period “full” of voltaic charge. Each
time a photon strikes the well, a bit of that charge is
depleted. The longer the integration time, the more
photons can be collected at each pixel; however, once
the charge is fully depleted, that pixel is “saturated”
and no new signal can be collected. In effect, each
photon is consumed as it strikes the detector and its
energy is released; we will not see it again.
10. Enter the Matrix
At the end of each integration period, the charge level
is read from all pixels on the detector. This read-out (still
in analog volts) is passed into an ADC (analog-to-digital
converter), which converts each pixel’s voltage into a
specific number of “counts.” This is when the photon
becomes data. From this point onward, we’ll be following the digital “intensity counts” (or spectrum) as it
moves through the electronics.
11. Dynamic Range
When converting the analog voltage of each pixel into a
discrete quantity, the resolution of the ADC plays a part
in determining the “dynamic range” of the spectrum. A
12-bit ADC can only represent values from 0 to 4095
(212), so the greatest “range” between the highest peak
and the baseline is 4096 counts. In contrast, a 16-bit
ADC can show discrete wavelength intensities from 0 to
65,535 (216), and an 18-bit ADC can provide even more
“vertical” resolution.
12. µC to PC
The spectrum is then copied over USB from the spectrometer’s microcontroller (a Cypress FX2 chip) to the
host PC over USB. An HR2000+ spectrometer, for example, would transmit an array of 2048 numbers, each an
integer from 0 to 4095. Different USB versions transmit
data at different speeds, which can affect the “scan
rate” (number of spectra that can be read per second).
Also, there are other communication buses available
besides USB, including RS-232, Bluetooth, Ethernet and
Wi-Fi, each with its own advantages and pitfalls.
13. The Final Drive
Reading a spectrum over USB doesn’t have to be complicated for the user, but there are a lot of things that
have to be done correctly. Fortunately, most of the very
low-level USB functions have already been automated
by low-level USB device drivers that come for free with
modern computer operating systems. At Ocean Optics,
we have also tried to automate most of the higher-level
spectrometer control functions using application
drivers like OOIDrv32, OmniDriver and SeaBreeze. With
a few lines of code, users will be able to read and
process spectra quickly and easily using their preferred
mix of drivers.
14. How Low Can You Go?
There are several good USB drivers for Windows,
including Cypress ezUSB and Microsoft’s own WinUSB
(best for 64 bit). Both Linux and MacOS use the
open-source libusb. Ocean Optics high-level drivers
“wrap” or “call into” these third-party drivers so that
you don’t have to learn their sometimes arcane conventions and idiosyncrasies (although you can if you wish!).
15. Application Drivers
OmniDriver, SeaBreeze and the original
OOIDrv32 driver they replaced provide
high-level function calls like getSpectrum() and setIntegrationTime() to
easily control your spectrometer and
retrieve data.
Either can be used from a variety of
programming languages and platforms
to quickly compile custom spectroscopy applications and GUIs. C, C++, C#,
Java, LabVIEW, and many other targets
are supported.
16. Post-Processing Spectra
After you have read your spectrum from a driver call,
you’ll probably want to clean up and filter your spectrum using a variety of common techniques:
www.oceanoptics.com | [email protected] | US +1 727-733-2447 EUROPE +31 26-3190500 ASIA +86 21-6295-6600
• Electrical Dark Correction (EDC): Subtract averaged “optically masked dark pixels” to correct for readout noise and thermal drift.
• Non-Linearity Correction (NLC): Apply a factory-calibrated 7th-order polynomial to correct for pixel
response as the detector’s wells approach saturation.
• Boxcar Averaging: Smooth out high-frequency
noise by averaging each pixel with its adjacent neighbors.
• Scan Averaging: Improve signal-to-noise ratio (SNR)
by averaging multiple spectra together.
17. Journey’s End
This concludes our photon’s journey. From its creation, to
its travels within the spectrometer, to its arrival at the
detector pixel cell, the photon navigates an elegant path.
As each photon’s “fate” is recorded and digitized into
data, additional stops along the way lead us to graphable
spectra in a format we can understand.
Contact us today for more information
on setting up your spectroscopy
system from Ocean Optics.
www.oceanoptics.com | [email protected] | US +1 727-733-2447 EUROPE +31 26-3190500 ASIA +86 21-6295-6600