Dust and dirt do not stand a chance – free view due to camera filters

TechTip: Free view due to camera filters
Dust and dirt do not stand a chance –
free view due to camera filters
To achieve optimum image quality the sensor needs to be kept free of dust. As
pixels today are the same size as dust particles or even much smaller, a single
speck of dust on the image sensor can drastically reduce the quality of the image.
The short distance to the pixel means that under certain lighting conditions a
speck of dust on the image sensor can cast the pixel beneath it entirely in shadow. The speck of dust appears as a dark spot in the camera image.
Background
The image sensor of a camera is an important component and requires protection
against contamination. Each sensor in an IDS camera has an adhesive-bonded
Schott D263 type sensor cover glass. This glass is opaque to wavelengths below
330 nm.
By default, another filter glass is used in front of the image sensor in all housed
cameras to prevent the intrusion of dust particles. The space in front of the image
sensor is protected against ingress of dust. Once the camera has been closed in
the IDS cleanroom production, no dust can get onto the image sensor.
Color cameras by default use an IR cut filter, which is required to ensure correct
color rendering. The default non-reflective filter glass in monochrome cameras
has no filter effect.
Spectral filtering of the incident light
The second purpose of the filter is the spectral filtering of light in front of the image sensor. Color camera pixels are also sensitive to light in the IR spectrum. If
IR light hits the image sensor as well as visible light, this leads to color distortions
and the image produced is dull.
Red, green and blue
wavelengths on the
example of UI-324x
For this reason, the detection of IR light is not desirable in most applications. IDS
Imaging Development Systems GmbH fits all color cameras with a filter that
blocks incident IR light as standard (HQ filter).
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TechTip: Free view due to camera filters
Filter type HQ
A different type of filter performs the exact opposite function. It lets IR light
through and blocks the visible portion of the light. This is particularly useful in applications in which work is carried out using IR lighting. Light from other light
sources is filtered out reliably by the daylight cut filter "DL Cut".
Filter type DL
Cameras with a so-called GL filter – a glass without any spectral filter characteristics – remain sensitive to the entire light spectrum.
The transmission curves for the various filters indicate which filter to use for a
specific application. In some exceptional cases, therefore, it may be useful to also
fit a GL filter to a color camera, for example.
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TechTip: Free view due to camera filters
Filter type GL
Camera filter in uEye Cockpit
When using color cameras with IR filter glasses, you need to set the appropriate
color correction matrix to ensure correct color rendering. The driver detects the IR
filter type and sets this value automatically. It is possible to set the sensor color
correction manually via the slider or to disable it entirely. In the “Color” tab of the
camera properties, you can change the settings via the “IR color filter correction
matrix” area.
Properties “Color”
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TechTip: Free view due to camera filters
Application areas
An argument for using an IR cut filter is that IR light has the effect of reducing the
color reproduction.
Color images with
and without HQ filter
Captures with daylight cut filter can be used to evaluate specific characteristics of
the photographed objects, for example to discover plant damages. Here, for example the effect is used that captures of healthy leaves look different than sick
leaves with IR light.
Color image with DL
filter
Summary
Filter glasses protect the sensor against dirt and allow a correct color reproduction. For special applications, cameras with different filter glasses can be used or
the color correction can be easily disabled in uEye Cockpit. Alternatively, the image acquisition with RAW format prevents any change of the pixel values.
Note the specific information in the uEye manual when you need to clean dirty filter glasses. Operate the camera only with built-in filters as dirt is hard to remove
from the sensor without using special materials. For this reason, the filter of the
new USB 3 uEye CP Rev. 2 cannot be removed with a special tool as it was previously possible for older C-mount models.
The table below gives an overview of the optical filters which are installed in uEye
cameras:
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TechTip: Free view due to camera filters
Filter type
Name
Refractive
index (nFilter)
Glass
type
Thickness
(f)
Cut-off
frequency
Non-reflective
IR cut filter
HQ
1.53
D263
1 mm
650 nm
On one side
Glass
GL
1.53
D263T
1 mm
330 nm
On both sides
Daylight cut filter
DL
1.53
RG695
1 mm
695 nm
-
For further information on camera filters, refer to the uEye manual at http://de.idsimaging.com/manuals-ueye.html.
Authors
Dana Diezemann, Senior Vision Consultant
Marion Gentele, Technical Documentation
Contact
IDS Imaging Development Systems GmbH
Dimbacher Straße 6-8
74182 Obersulm
Germany
Phone: +49 7134 96196-0
Email: marketing@ids-imaging.com
Web: www.ids-imaging.com
© 2015 IDS Imaging Development Systems GmbH
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