How Many Colors Can A Camera Recognize
Introduction
Life is total of colour, and capturing an image that replicates human being perception of color is an important, and sometimes challenging, aspect of both everyday life and scientific inquiry.
Although many cameras, such as phone cameras, video cameras, and commercial digital cameras, produce color images, a large portion of scientific enquiry is carried out using monochrome cameras. Monochrome cameras capture only the intensity of low-cal and not its color, yielding greyscale images that may be afterwards false-colored digitally.
Scientific color cameras are about oft used in fields such as life sciences to document observations, particularly in histology and pathology, where samples of cells or tissues are inspected to understand health and diagnose affliction. These types of samples crave colour imaging as tissue and cellular samples often consist of thin, transparent or semi-transparent sections devoid of pregnant contrast lone.
Since camera sensors lonely cannot 'see' unlike colors, to capture colour images, cameras must utilize a machinery to separate the cherry, green and blue color components of the light. Standard monochrome photographic camera sensors used in scientific imaging can exist modified to capture color images.
In that location are unlike methods available that permit scientific color imaging, each of which relies on the addition of components, typically during manufacture, to permit imaging with scarlet, green and blue. The important thing to call back is that all iii colors must be acquired per frame, so each method is generally accompanied by a trade-off past a factor of three in different aspects of camera functioning.
In this technical note, we will discuss why colour imaging requires a cede by a gene of iii, how colour imaging is achieved and how to best select the right color imaging technique for your desired application.
What Is Color?
To sympathize color imaging, it is offset essential to understand visible light. Visible light consists of two major components, wavelength and aamplitude, which correspond color and brightness respectively (Fig.1). White light contains all the wavelengths of visible calorie-free at approximately equal intensities, whereas light sources that appear only blueish or only red consist of mainly the wavelengths respective to these regions.
A color image is a digital prototype that includes intensity information for dissimilar colors at each pixel. Although color is a continuous spectrum, colors are ofttimes described using the reddish, green, blue (RGB) model. The RGB model is representative of how human perception of colour works by using three different types of cones in the centre that are sensitive to red, green and blue. Combinations of these three main colors tin exist combined to create a wide range of colors seen in everyday life. This model is as well used for digital cameras, monitors and paradigm file formats.
What Gives Ascent to Colour?
Objects appear in color due to their interaction with the different wavelengths of low-cal. The interactions that tin occur between light and matter are broken down into absorption, manual and reflection.
When objects are perceived as a colour (i.e. bluish) it is because they are reflecting or transmitting blue wavelengths of light towards our eyes and absorbing the other wavelengths (i.e. green and blood-red).
There are three main aspects of colour imaging:
- Shining light towards a sample
- The wavelengths of light interacting with the sample to different degrees based on local properties
- The detection of the reflected wavelengths of light from the sample (cerise, greenish and blueish components)
The light used in color imaging is normally white lite. The white low-cal is directed towards a sample and will interact with the sample. The sample itself volition contain many atoms and molecules and each will have associated electrons. If the light has a wavelength that matches the vibrational frequency (resonance frequency) of the electrons, and so that wavelength will be absorbed and converted into vibrational and thermal free energy. Equally different atoms and molecules have different natural vibration frequencies, they will selectively absorb different frequencies of visible lite.
Likewise, those wavelengths that do not match the vibrational frequencies of the atoms and molecules so the wavelengths of light will either pass through the material and be transmitted, or they volition be reflected or scattered. This interaction, of either assimilation, transmission or reflection decides which light waves are directed towards our eyes, or a scientific camera, determining the color that an object will appear. If many wavelengths are transmitted, then the object will exist transparent, whereas if they are reflected they will appear opaque.
Once the light has interacted with the sample, the light reflected or transmitted can be detected past a digital photographic camera (or our eyes). One time acquired, the 3 different color channels are recombined to give a full-colour image, where each pixel has an RGB intensity value, which together tin can make any combination of visible colors.
Staining Methods For Color Imaging
Many thin sections or samples in biological imaging are devoid of color. To assistance biological research and meliorate contrast, samples can be stained with dyes that change the absorption profiles of structures within the sample that the dye binds to. This leads to the transmission and reflection of different wavelengths and therefore modifies the contributions in the red, green and blue channels and intensities, highlighting various cellular architecture.
Staining requires a dye that binds to certain components of the cells, giving them contrast compared to the unstained (or counterstained) groundwork. This staining can either be selective and specific, i.eastward. for chemic groups, or can be non-specific, staining most of the prison cell. Often staining regimes include a specific stain, followed past a non-specific counterstain. The intensity and appearance of stains can exist quantified following imaging, which can be used to identify subtle changes not detectable to the man middle.
This forms the basis of many diagnostic or monitoring methods. In histopathology, differential staining patterns of samples can aid disease diagnosis based on the organisation (or lack thereof) of the cells and can highlight abnormalities such as changes to nuclear or cytoplasm morphology that occur during cancer.
Types Of Color Cameras
At the virtually basic level, color cameras work like conventional monochrome cameras, meaning that incoming photons interact with calorie-free-sensitive pixels during exposure, irrelevant of wavelength or color, and the intensity of each pixel is read out as a digital image. Monochrome cameras, without additional components such as filters, do not differentiate betwixt different wavelengths of calorie-free.
When RGB images are desired, the incoming white light must be separated into its component colors, each of which is independently measured. There are four main methods of achieving this:
- Using filter array masks on the detector itself splitting upwards the detector into dedicated red green and bluish pixels
- Using moving filters in the light path and acquiring separate images
- Using three unlike sensors, ane for each colour
- Using a fast switching light source
These methods of converting a standard monochrome camera into a color photographic camera are detailed in the next sections.
Colour Filter Arrays
Colour Filter Arrays (CFA) are the industry standard for coloring imaging. CFAs are used to mask unlike pixels on an assortment. The most mutual blazon of CFA is the Bayer filter array, which arranges pixel filters in a GRBG pattern (2×two), as shown in Fig.2 repeated across the unabridged detector. In this method, each pixel corresponds to a colored filter and is dedicated to imaging this colour only.
At each pixel, the filter prevents the passage of light that is non of the desired color, so a pixel defended to measuring reddish light, simply permits cerise wavelengths to laissez passer through. Following passage through the filter to the defended pixels, photons are detected by the light-sensitive region as in conventional scientific camera lite drove.
This Bayer filter seen in Fig.2 has twice as many light-green masked pixels as red or blue. This is done because the human middle is more than sensitive to green lite, and therefore green pixel redundancy produces an image that appears less noisy and has finer details than could be accomplished if each colour were treated equally.
As the exact location of each filter is known, following the conquering of the cherry-red green and blue images, an algorithm can interpolate between pixels to create an image that has RGB intensity values. However, equally each pixel is only really acquiring data for crimson, greenish or blue, this ways ii/3 of the pixel intensities are interpolated, decreasing the resolution by a factor of three.
Sensitivity is also affected, as only a fraction of the available photographic camera pixels can be used to discover light of a given wavelength. Just 25% of the pixels of the sensor tin can detect cherry-red or blueish light, and just 50% for light-green.
The reward of using a Bayer filter is speed. When using the Bayer filter cameras, just one acquisition is required and simply one sensor is needed. This is advantageous as all colors are acquired at the same time. Imaging is, therefore, faster than acquiring three sequential images and does not adventure loss of temporal information or simultaneity if imaging live or at high speeds. Additionally, the simplicity and cost-effectiveness of having a single sensor and no moving parts make this the most pop method for color acquisition.
The major downside to this method of color prototype capture is the loss of resolution and sensitivity due to the interpolation and binning of neighboring pixels which is unavoidable due to the pixel masking. This can also lead to interpolation errors and loss of information. Another disadvantage is that CFAs make utilise of absorptive dye filters, which are inferior to dichroic filters in terms of colour purity.
Moving Filter Wheels
An alternative to masking pixels with a Bayer filter is to use multiple acquisitions that are taken individually. The chief option hither involves the employ of moving filters to sequentially acquire each aqueduct image.
The method of moving filters in the light path with a standard monochrome photographic camera permits sequential filtering for red, green and and so blueish light imaging in the lite path. Once all three acquisitions are complete, the three images are recombined to make the total-color paradigm, with each pixel having a RGB value (Fig.3).
This sequential conquering of each channel allows the unabridged sensor area to be used so that all available pixels are collecting either red light, light-green light or blue lite. This method permits a higher resolution than using Bayer filters equally intensities for each color epitome is acquired at each pixel. Therefore, no information is lost, and no interpolation is necessary to permit the images to be combined.
During sequential acquisitions, it is sometimes thought useful to modify the exposure time, based on the detector and the wavelengths being imaged, offering additional flexibility compared to the CFA method. Each pixel in the CFA is subject to the same exposure time, as they are acquired together, but there is no such limitation with this technique.
However, there is a large drawback in the moving filter method, in that the time taken to switch the filters inside the system reduces the speed of imaging by 67%. If speed is not important then this could be the best method, nevertheless where speed is important it would lead to loss of temporal resolution. Not only that, the images would be taken at subtly different times every bit they are acquired sequentially. Due to the diverse mechanistic problems with moving filter wheels, these are non regularly integrated into consumer-grade cameras.
Three Cameras Or Sensors
There is another alternative that does non crave any filter switching nor loss of FOV or resolution. This option involves the employ of three separate cameras or three separate sensors inside a single photographic camera.
In this case, each photographic camera or sensor is dedicated to imaging a specific color (either cherry-red, green or blue). In this set up, post-obit sample illumination, a axle splitter can exist used to separate the light into three separate beams (Fig.4). The beams then laissez passer through various dichroic filters, separating the light into narrow bands which tin and so be detected on the respective camera or sensor.
One method of incorporating three sensors in ane photographic camera can be based on using silicon sensor stacks of multiple photodiodes to detect each color at each pixel (Fig.4B). A 3-sensor stack relies on the wavelength dependence of photon assimilation, and the charge is collected separately at unlike depths in the silicon, all the same, this technique is more challenging to perfect and withal under development.
This method offers a 1:i colour ratio which means that 3 sensors offer the highest paradigm quality, whilst maintaining the whole field of view for each color aslope no loss in speed for filter changing. The image quality is also improved due to the use of dichroic filters, which are by and large of college quality than those used in the Bayer masking. This also ways that no interpolation is necessary, as no pixels are lost, which improves sensitivity and resolution.
The most obvious downside of this set upwards is that, due to the need for three dissever sensors/cameras, they too tend to cost significantly more coin (roughly three times more) than other color camera solutions. These types of camera should be picked when an verbal RGB value and the best image quality is required.
Fast Switching Light Source
A new solution has recently become bachelor in the form of the fast switching light sources which circumvent some of the issues related to errors in interpolation, loss of resolution, speed merchandise-off and increased cost.
A light source that tin switch between red, green and blue excitation light rapidly tin exist combined with a monochrome camera to permit color imaging. These types of light sources often include solid-state LEDs with a fast colour-switching mechanism included that permits that acquisition of RGB channel images. The acquisitions are generally captured in a high-speed sequence through an internal camera trigger, allowing loftier-speed epitome acquisition without the need for moving filters.
This method removes the variance associated with moving components such as filters and permits the perfect registration of pixels in the image across a big field of view. The calorie-free source method, therefore, improves upon the speed, sensitivity, and precision of colour transmitted light microscopy. The light source besides offers a cheaper alternative to using three cameras, whilst maintaining the advantages offered by this method.
However, the light source still requires time to switch the calorie-free sources, so although faster than switching color filters, information technology even so is slowed from the potential maximum rate by a factor of 3 to incorporate this switching mechanism.
Summary
The relative advantages and major disadvantages of each of the color imaging methods are summarized in Table one. The right solution for the individual application will vary depending upon which factors are most important for the imaging arrangement and requirements.
Table i: Summary of the potential mechanisms for collecting color images and their primary reward and disadvantage.
| Camera Type | Mechanism | Advantages | Disadvantages |
| Bayer Filter Array/Mask | Masking of pixels | Fast, cheap | 33% resolution and sensitivity |
| Moving Filters | Filter cycle | Max resolution | 33% speed |
| Three Sensors/Cameras | Separate sensors for RGB | Max resolution and fast | 3x price |
| Switching Light Source | Internal trigger LED | Max resolution | 33% speed |
The conquering of color images is an essential aspect of many biological imaging investigations. At that place are a few distinct methods that can be employed to obtain these iii-color images, each with associated advantages and disadvantages that are presented in Table 1. Generally, there is a merchandise-off either in the resolution obtained in the images, the time taken to larn the images or in the cost of the organisation.
The cheapest method remains to use a defended color imaging device that contains a Bayer mask array within. If the camera is likely to have a dual purpose for fluorescent imaging, or if the loss of spatial resolution is more of import than collecting simultaneous images information technology may be worthwhile to buy a monochrome camera and use a mechanical filter wheel to sequentially acquire the images.
The highest quality information will be obtained when collecting the images with a three sensor/camera option that permits fast, simultaneous imaging of each channel with the full field of view of the sensor, requiring no interpolation. Withal, this selection will be the most expensive equally the cost is finer three times greater due to the cost of the sensors, and the range of commercially available cameras that implement this engineering is poor.
Alternatively, to achieve a balance of resolution, spatial and temporal, likewise as minimizing costs and maximizing epitome quality and facilitating easy realignment, an internally triggered fast switching LED calorie-free source linked with a monochrome camera may offer the best of both worlds for color imaging applications and maintaining the advantages of a scientific monochrome photographic camera for fluorescence imaging.
Source: https://www.photometrics.com/learn/microscopy-basics/imaging-in-color
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