Section 4.2: Sensor Relative
Illumination, Roll-Off, and Vignetting
In order to evaluate and understand issues that can be associated
with vignetting (the blocking of rays that pass through the outer
edges of an imaging lens), roll-off and relative illumination, sensor
24 +44 (0)1904 788600 • EDMUND OPTICS® imaging lenses filters microscopy cameras illumination targets imaging resource guide
sizes, and formats need to be considered. In addition to the following
overview, more information on sensors and formats can be
found on pages 148-151.
Matching Sensors with Lenses
One issue that often arises is the ability of an imaging lens to support
certain sensor sizes. If the sensor is too large for the lens design,
the resulting image may appear to fade away and degrade towards
the edges; this effect is caused by vignetting. As the resolution
demands of a system increase, one of two things needs to happen:
pixels need to get smaller or sensors need to get bigger. As is detailed
in the sections on MTF and diffraction limit (Sections 3.2 and
3.1), continuous reduction in pixel size produces significant issues
regarding the optics’ ability to resolve true detail. This concern,
combined with the signal to noise and sensitivity issues associated
with current sensor technology, results in sensors having to grow in
size; such growth causes these issues regarding vignetting and rolloff,
unless the proper lens is used.
Relative Illumination (RI) is a way of representing the combined effect
of vignetting and roll-off in an imaging lens, and is generally given
as a percentage of illumination at any point on the sensor, normalized
to the position in the field with maximum illumination. Vignetting
and roll-off are two separate components of RI. An example of RI is
shown in Figure 4.5. More details on how to read this curve can be
found in Section 2.6.
The curve in Figure 4.5 shows that at its lowest aperture setting
(f/1,4 – the blue line), this lens has an RI of 57% of the illumination
level of the image center at the corner of a 2/3” sensor. Under the
same conditions, the lens has an RI of 72% in the corner of a 1/2”
sensor. As the sensor gets smaller, the RI improves. Also note that
the RI improves as the f/# is increased; this occurs until there is no
more vignetting in the lens, at which point all higher f/# settings will
have the same illumination profile. Increasing the f/# generally does
not increase the image circle size much, in that a lens designed for a
particular sensor size will not perform well on a larger sensor even
with the f/# stopped down.
Roll-off will still occur when the lens is stopped down, as that is
related to the angle of the rays and not the number of rays that pass
through the lens. Many lenses will have an illumination profile that
is highest in the middle of the field and is either flat or declines to
some lower percentage as it approaches the edge. There are rare
cases where the RI increases slightly across the image circle, but this
is related to pupil compression and will not be covered in this text.
Vignetting Within a Lens – ADVANCED
Vignetting is the result of light rays not making it through the entire
lens system to the sensor, due to being blocked by the edges of individual
lens elements or mechanical stops. This clipping of rays can be
intentional or unintentional, and in some case is unavoidable. Vignetting
is most often seen at or in lower f/#s, short focal length lenses, or
lenses where higher resolutions need to be achieved at a lower cost.
Figure 4.6 demonstrates clipping as it may occur for the same
16mm lens at different f/#s (f/1,8 and f/4). Note the clipping of rays
in Figure 4.6a, as indicated with red circles; these rays are not able
to pass through all of the optics in the lens. Figure 4.6b, on the other
hand, demonstrates an example without vignetting. The vignetting in
Figure 4.6a could have several causes, including diameter limitations
of the optics or a need to eliminate the rays to block stray light. Vignetting
is sometimes purposely included in a lens design to improve
overall lens performance or reduce cost.
Roll-off is the decrease in relative illumination with respect to
field that is not caused by vignetting, but by radiometric laws.
0.00 1.50 3.00
0.00 2.75 5.50
Image Height (mm)
Sensor Format: ¼” ” ½” ”
Figure 4.5: A relative illumination curve showing relevant image sensor
formats on the x-axis.
Image Height (mm)
Figure 4.6: A 16 mm lens design at f/1,8 (a) and f/4 (b). At f/1,8 vignetting
occurs where light rays are clipped by the edges of the lens.