Advances in LED illumination
LEDs
An LED is a semiconductor diode that emits light when a current passes through it. As a diode current can only
flow in one direction. When electrons combine with the positive holes in the semiconductor, light is emitted. The
band-gap of the materials used to create the p-n junction defines the energy, and therefore wavelength, of the
emitted light. Some LED lights used phosphor coatings to alter the colour of the light from that emitted from the
diode. For example white LEDs mix yellow and blue light to give the appearance of a white light. The general
development of LED technology has shown an exponential increase in brightness – giving a doubling of
brightness every 36 months since the 1960s.
Figure 1 – diagram of an LED
The general options for illumination are LED, fluorescent and incandescent lights. Incandescent lights work by a
current heating a filament until it glows white hot, fluorescent lights by creating an arc (spark) to create light.
W W W . S T E M M E R - I M AG IN G . C O M ● IM AG IN G I S O U R P AS S IO N
GERMAN Y
UNIT ED KINGD OM
FR ANCE
SWIT ZERLAND
STEMMER IMAGING GmbH
Gutenbergstr. 9-13
82178 Puchheim
Phone: +49 89 80902-0
Fax:
+49 89 80902-116
[email protected]
STEMMER IMAGING Ltd.
The Old Barn, Grange Court
Tongham, Surrey GU10 1DW
Phone: +44 1252 780000
Fax:
+44 1252 780001
[email protected]
STEMMER IMAGING S.A.S.
23 bis, rue Edouard Nieuport
92150 Suresnes
Phone: +33 1 45069560
Fax:
+33 1 40991188
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STEMMER IMAGING AG
Rietbrunnen 48
8808 Pfäffikon SZ
Phone: +41 55 4159090
Fax:
+41 55 4159091
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LED Illumination – White LEDs
Colour
Single-wavelength LEDs can be very useful for imaging: narrow-band wavelength emission means that a better
focus is generally achievable and also makes it easy to filter out ambient light. However, when good colour
representation is required it is necessary to use a good white light source.
To give the best colour representation it is necessary that all visible wavelengths are emitted from a light source.
There is also an argument that infra-red should not be present as CCD cameras are sensitive to these
wavelengths beyond the visible spectrum.
Figure 2 – relative sensitivity to wavelength of a CCD sensor vs the human eye.
Nominally white fluorescent lights generally emit more strongly in the blue part of the visible spectrum (and UV).
Incandescent lights emit a smooth (black-body) spectrum centred on the 575nm wavelength. This is close to the
spectrum of sunlight and gives good colour representation to a human, but a significant IR portion is also
present. Normal white LEDs have a strong peak towards the blue end of the visible spectrum, making these not
ideal for good colour representation.
Figure 3 – comparison of the spectra of red and standard white LEDs
WW W. S TE MM E R - I M A GI N G. C OM
GERM AN Y
Phone: +49 89 80902-0
[email protected]
●
IMAGING IS O UR P ASSI ON
UNI TED KI N GD OM
Phone: +44 1252 780000
[email protected]
FRAN CE
Phone: +33 1 45069560
[email protected]
S WI TZE RL AND
Phone: +41 55 4159090
[email protected]
LED Illumination – White LEDs
However, a new development from CCS has produced ‘natural light LEDs’. These give a more even spectrum
than traditional white LEDs with the following features:
• Colour representation similar to sunlight for visible wavelengths (the colour temperature is 5000K,
sunlight is around 6000K).
• Minimal UV and IR (sunlight and incandescent lights contain significant amounts of IR)
Figure 4 – CCS CNR-110NW ‘Natural Light LED’ ringlight, designed for use with stereomicroscopes.
The effect of these two features is that very good colour representation is possible, but the absence of IR
reduces the red skew that is sometimes present in camera images. Also for direct viewing applications, such as
microscopy, lights with a strong blue component can cause more eye-strain. For sensitive targets such as
biological samples or PCBs it may also be useful to have no UV and IR present.
Figure 5 – spectra for sunlight, fluorescent white LED and ‘natural light LED’ sources.
WW W. S TE MM E R - I M A GI N G. C OM
GERM AN Y
Phone: +49 89 80902-0
[email protected]
●
IMAGING IS O UR P ASSI ON
UNI TED KI N GD OM
Phone: +44 1252 780000
[email protected]
FRAN CE
Phone: +33 1 45069560
[email protected]
S WI TZE RL AND
Phone: +41 55 4159090
[email protected]
LED Illumination – White LEDs
Figures 6 and 7 show comparisons of natural light LED illumination compared to fluorescent and halogen
sources. In both cases it can be seen that the colour representation and contrast is improved under natural light
LEDs.
Figure 6 – a comparison of colour representation of a flower under natural light LED (left) and fluorescent
illumination (right)
Figure 7 – Natural light LEDs for endoscopy applications. These are endoscope images of the human large
intestine illuminated the natural light LED (left) and a halogen source (right). Note the blood vessels are much
clearer on the left image.
Dr Jon Vickers, STEMMER IMAGING
WW W. S TE MM E R - I M A GI N G. C OM
GERM AN Y
Phone: +49 89 80902-0
[email protected]
●
IMAGING IS O UR P ASSI ON
UNI TED KI N GD OM
Phone: +44 1252 780000
[email protected]
FRAN CE
Phone: +33 1 45069560
[email protected]
S WI TZE RL AND
Phone: +41 55 4159090
[email protected]