Microscope - Digital Camera & Video Basics
Part 1 - What You Need to Know
What elements make a quality image?
- Resolution (optical)
- Resolution (electronic)
- Dynamic range (ISO / Sensitivity)
- Noise (lack of…)
- Method of reproduction (print / screen)
As far as the digital camera is concerned, there is a set process on how we receive the end result of an image.
Light > Lens > Filters > Sensor > ADC > Output
We have all heard of the word “pixels”. We can get very tied up with this and sometimes decisions are made based on whether or not this number is high enough or not. This should not be the only contributing factor when making a decision on what type of camera is best suited.
What are pixels?
A pixel is a coloured dot that makes up a digital image. A digital image is basically a mosaic of square pixels of uniform colours which are so tiny they appear uniform and smooth.
What is resolution?
It is the ability to discriminate between two objects and to determine contrast. Quality resolution is mainly the function of the lenses and is called optical resolution. There is a limit to optical resolution that must be taken into account.
Terms like “sharpness” and “contrast” are properties of the lenses and not a function of the camera as such.
Electronic resolution is the total number of pixels used in a chip or sensor sorted in a square or rectangular array. E.g. 1000 x 1000 = 1,000,000 megapixels (MP)
Each pixel can only have one colour and have one brightness value.The more detail you have, the more you can blow up a picture before it becomes ‘grainy’ and start’s to look out of focus. This is really only important if you want to print photos. The downfall of more pixels and therefore resolution is bigger files, which leads to more storage space and more computer processing power is required.
What is a “Chip”?
The Chip or Sensor in digital cameras is a semiconductor device that records light (brightness and colour) as electronic information. The two devices or chip types used are: -
- CCD - Charged Coupler Device
- CMOS – Complementary Metal Oxide Semiconductor
The main differences between the two technologies are manufacturing, power consumption, noise and image processing.
CCD
- High quality, low noise images
- Generally more pixels
- Uses up to 100 times more power than CMOS chip
- Are of higher quality due to older technology and greater understanding of mass production issues
CMOS
- More noise due to more transistors surrounding each pixel
- Consumes less power. Less pixels, therefore less processing power required
- Can be made on simpler more basic silicone production lines and therefore a lot cheaper to manufacturer and buy
In a CCD device, the charge after light has exposed the pixels is actually transported across the chip and read at one corner of the square or rectangular array.
The CMOS chip design allows each photosite (pixel) to read individually. There are several transistors in each photosite that amplify and move the charge using more traditional wires.
The Chip or Sensor is a collection of tiny light sensitive diodes which convert photons into electrons (electrical charge). These diodes are called “photosites”. Each “photosite” is sensitive to light. The brighter the light that hits a photosite the more electrons accumulate at that site. Photosites come in various sizes from 10-11 microns for SLR cameras down to 3-4 microns for domestic grade small digital cameras.
Is having a bigger “Photosite” better?
Yes it is and it is the area of the photosite that is important. A 3 micron photosite has 9 square microns of area, whilst an 8 micron photosite has 64 square microns of area.
Bigger photosites = bigger pixels =better resolution
“Photosites” and Sensors are colour blind
Each individual photosite simply collects only the amount of light hitting it and passes that data on. No colour information is collected so the bare chip or sensor is of monochromatic design.
The chip or sensor gets it colour from: -
- Filters/beam splitters
- Primary colours (Red, Green & Blue)
- Bayer filter
What does the sensor array look like?
Does the filter quality matter?
- Yes - filter quality is proportional to colour quality.
- Filter consistency = consistent colour
- Some sensors have the filter pattern incorporated into the chip
- Some microprocessors “help” colour
Remember: As the chip or sensor is made up of light sensitive diodes called photosites, the size of the photosite is important.
Remember: Electronic Resolution = number of pixels (Photosites)
E.g. a SLR camera has 16.6 EMP (effective mega pixels) = 17.2 micron square sensor photosite.
It is not digital yet?
Light is converted into an electric charge, but the electrical charges that build up in the sensor are not digital signals. In order to digitise the information, the signal must be passed through an analog-to-digital converter (ADC).
Sampling Rate
The analogue signal is continuous in time and it is necessary to convert this to a flow of digital values. It is therefore required to define the rate at which new digital values are sampled from the analog signal. The rate of new values is called the sampling rate or sampling frequency of the converter.
“Effective” megapixels!!
Most sensors are measured in effective megapixels (EMP) as not all the sensor area is used for collecting information. Some of the photosites are blacked out e.g. 1000x1000 = 1MP = 0.8EMP. It gives the ability for the sensor and processor to compare light and dark pixel values for exposure settings and aids in the de-mosaicing and interpolation of the image.
Megapixels, is more better?
The marketing race for more pixels would like us to believe that more is better, but it is not that simple. The number of pixels is only one of many factors affecting image quality. We must take into account the quality of the pixel itself such as the geometrical accuracy, colour accuracy of the filter, dynamic range, noise and artifacts. The optical system attached (optical resolution) to the camera and microscope is as important, if not more important when assessing resolution/sharpness and must always be considered as well.
Conclusion
I hope you have enjoyed Part 1 of this greatly reduced technical topic, Digital Camera & Video Basics, and see the relevance of how “you cannot manage you cannot see!”
Next month we will look at the final step of digital processing, which is image abnormalities, output and screen and print resolution. Please feel free to send through any information that you would like to contribute in regards to this topic and we will be sure to incorporate it where possible.
Phone: 1300 033 723
Email : peter@inline.com.au
Upgrade to the NEW Ultra LED from GLOBAL today - $2300 + GST Limited Time Only
Phone: 1300 033 723
Email : peter@inline.com.au