Wood Rocks Rocks
380 430 480 530 580 630 680
5000 K – 8300 K CCT
3700 K – 5000 K CCT
2600 K – 3700 K CCT
Relative radiant power
While the lightbulb has a similar shape curve
as the sun, it’s lower temperature yields less overall emitted light at any given wavelength. More
importantly the light bulb has more photons
in the red band compared to blue and green.
That’s why objects appear redder indoors then
when illuminated by the sun where the ratio
of red photons to blue or green is very different. This is why objects appear different indoors
because the combination of wavelengths emitted from the bulb are different from the sun, so
the amount of light reflecting off of the object
will also be different.
Incandescent bulbs such as this 60W bulb
are no longer readily available, and most mod-ern-day machine vision illumination is provided by Light Emitting Diodes (LEDs). LEDs
are semiconductors that emit a specific wavelength of light or a very small range of wavelengths of light, usually + or – 50nm (with
many high end LEDs , +/- 15nm) around a
In the visible region we have developed
many types of LEDs that provide the correct color illumination for the scene. Lights
for machine vision are generally specified by
color desired or color temperature desired (red,
green, blue, or white at 6000K, etc.).
6000K white light
Lux or illuminance specifies light brightness,
or how many photons a white light visible
source emits. For single- or multi-color LED
sources, light is specified in m W/cm2, and lux
is a measure of white light. By combining mul-
tiple LED colors, white light or various color
temperatures of light can be simulated. Figure
5 is the spectrum of a 6000K white light from
Metaphase Technologies (Bristol, PA, USA;
www.metaphase-tech.com). This is simulating
a 6000K blackbody in the visible band, where
it is very effective.
However, visible LED illumination is completely ineffective for the new generation of
SWIR machine vision cameras. Such cameras typically range in sensitivity from 400nm
to 1700nm or 1000nm to 2600nm depending
on the type of image sensor used. In the past
a 6000K blackbody would be a very effective
broadband source for these cameras but today’s
white LED sources generally do not provide
any light beyond 785nm.
The machine vision lighting industry
has developed NIR sources that generally
use LEDs of 950nm or 850nm wavelengths,
New, low cost SWIR linescan cameras
made with InGa As image sensors can see vis-
ible light as well as SWIR light from 400nm
to 1700nm. As mentioned previously, incan-
descent lights can be used with these cameras
because they are essentially blackbody sources
that emit a broad range of wavelengths from
400nm to more than 2500nm depending on
the glass used to encase the bulb.
However, since the market has moved to
LEDs, most sources are very specific in wavelength with a bandwidth of +/-50nm typically.
While one may consider these white light visible LED sources as broadband in the visible spectrum, because they are a combination of many LED colors, they will most likely
not be broadband for the NIR and SWIR
Now that Vis-SWIR detector systems like
the LineCam12 from Princeton Infrared Technologies Inc. (Monmouth Junction, NJ, USA;
www.princetonirtech.com) are available, light
source selection must consider wavelengths
beyond the usual Red, Green, Blue, Ultra-Vio-let, and NIR sources at 850 or 950nm.
Figure 6. Coffee beans, rocks and sticks imaged with a visible camera and SWIR camera with
1450nm light. The objects are all shades of brown making differentiation difficult in the visible
but very simple in the SWIR where the coffee beans are very reflective.
Figure 5. Wavelength output of various Metaphase LED white light sources. The LEDs attempt to
simulate a blackbody at those temperatures by shifting the peak wavelength.