Depending on an application’s requirements, a camera’s ability to reproduce
targets illumination microscopy filters imaging lenses imaging resource guide 150 +44 (0)1904 788600 • EDMUND OPTICS® cameras
color may or may not be beneficial. A comparison of monochrome,
single chip color, and three chip color cameras is shown in
Table 4 and more detail is provided in the following section.
monochrome VS. color
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Monochrome Color (Single Chip) 3 Chip Color Cameras
• Single Sensor Outputs Grayscale Images • Uses RGB Bayer Color Filter (Typical) • Utilizes a Prism to Split White Light into 3 Different Sensors
• 10% Higher Resolution than Comparable Single-Chip Color Cameras • Lower Resolution (More Pixels Required To Recognize Color) • More Costly
• Better Signal-To-Noise Ratio; Greater Contrast • Better Color Resolution
• Increased Low-light Sensitivity • Smaller Selection of Lenses
• Mag Require Specially Designed Lenses
Table 4: Monochrome vs. Color.
CCD and CMOS sensors, being silicon devices, are sensitive to wavelengths
from approximately 350 - 1050 nm, although the usable range
is usually given from 400 - 1000 nm. This sensitivity is indicated by
the sensor’s spectral response curve (Figure 2). However, most highquality
color, and some monochrome, cameras provide an infrared
(IR) cut-off filter for imaging specifically in the visible spectrum.
The solid state sensor is based on the photoelectric effect and, as a
result, cannot distinguish between colors without additional considerations.
There are two types of color CCD cameras: single chip and
three-chip. Single chip color CCD cameras offer a common, low-cost
imaging solution and use a mosaic (e.g. Bayer) optical filter to make
different pixels sensitive to only certain wavelengths of light. A color
image is then reconstructed in software using a “de-bayering” algorithm
which interpolates true color information from the RGB signals.
Since more pixels are required to recognize color, single chip color
cameras inherently have lower resolution than their monochrome
counterparts. Three-chip color CCD (3CCD) cameras are designed to
solve this resolution problem by using a prism to direct each section
of the incident spectrum to a different chip. Although 3CCD cameras
typically provide extremely high resolutions and more accurate color
reproduction, they have lower light sensitivities and can be costly.
Frame Rate and Shutter Speeds
The frame rate refers to the number of full frames composed in a second.
In high-speed applications, it is beneficial to choose a faster frame
rate to acquire more images of the object as it moves through the FOV.
The shutter speed corresponds to the inverse of the exposure time
of the sensor. The exposure time controls the amount of incident light
collected by the sensor. Camera blooming (caused by over-exposure)
can be controlled by decreasing illumination, or by increasing the
shutter speed (decreasing exposure time).
The maximum frame rate for a system depends on the sensor readout
speed, the data transfer rate of the interface, and the number of
pixels (amount of data transferred per frame). Often, a camera may be
run at a higher frame rate by reducing the resolution by binning pixels
together or restricting the area of interest. For digital cameras, exposures
can be made from tens of microseconds to minutes, although
the longest exposures are generally only practical with CCD cameras,
which have lower dark currents and noise compared to CMOS.
Electronic Shutter: Global vs. Rolling
A global shutter is analogous to a mechanical shutter, in that all pixels
are exposed and sampled simultaneously, with the readout then occurring
sequentially; the photon acquisition starts and stops at the
same time for all pixels. On the other hand, a rolling shutter exposes,
samples, and reads out sequentially; it implies that each line of the
image is sampled at a slightly different time. Intuitively, images of
Normalized Response of a Typical Monochrome CCD
with IR cutoff filter without IR cutoff filter
400 500 600 700 800 900 1000 1100 1200
Relative Spectral Response
Figure 2: Normalized Spectral Response of a Typical Monochrome CCD.
A: Stationary B: Global Shutter
C: Rolling Shutter
Figure 3: Comparison of Motion Blur. Stationary PCB (A) and images of
moving PCB with Continuous Global Shutter (B) and Rolling Shutter (C).
moving objects are distorted by a rolling shutter; this effect can be
minimized with a triggered strobe placed at the point in time where
the integration period of the lines overlaps. Note that this is not an
issue at low speeds. Implementing global shutter for CMOS requires
more complicated sensor architecture than the standard rolling shutter
model, and thus they are not available on all CMOS sensors. A
comparison of global and rolling shutters is shown in Figure 3.
In contrast to global and rolling shutters, an asynchronous shutter
refers to the triggered exposure of the pixels. That is, the camera
is ready to acquire an image, but it does not enable the pixels until
after receiving an external triggering signal. This is opposed to a
normal constant frame rate, which can be thought of as internal
triggering of the shutter.