Colposcopy commonly uses a green filter to highlight blood vessels in the tissue. Because many indicators of pathology are related to blood flow, the green filter enables a clearer identification of disease. MobileODT’s mobile colposcope, the EVA System, uses a digital colposcope green filter to present images. As no industry standards exist for a colposcope green filter, the EVA System green filter results from in depth research.
Why is a colposcope green filter important?
Blood has a red color because it absorbs green and blue light, but reflects red. A green filter greatly reduces red coloration within an image, and areas with visible blood vessels turn dark. Therefore, when looking at the image of a cervix, the blood filled red areas appear black or darker in color when seen through a green filter while pink tissue is less affected and comes off as bright green, enabling colposcopists to identify blood vessels more easily. Clinicians can then use the blood vessel pattern to differentiate between areas of specific pathologic features and unaffected sites.
Colposcopes have incorporated green filters for many years. Traditional colposcopes require clinicians to insert a green slide over the lens, bathing the image with green light. As digital colposcopes have been manufactured, digital methods of applying, such a green filter, have also been developed.
An image of a cervix with coarse mosaic, captured using the EVA system. On the left a regular white light image, in the middle EVA’s new green filter, on the right an “no red” filter similar to some commonly used colposcopes.
3 ways of producing a green filter
Producing a green filtered image requires increasing the intensity of the green light (500-570 nm), compared to the red and blue, from within the image before viewed by the clinician. This can manifest in multiple ways, either at the point of illumination, at the point of image collection, or digitally, before viewing the image.
FILTERING ILLUMINATION
The light source used when taking the picture is adapted so that only green light shines on the cervix as the picture is being taken. Because only green light illuminates the cervix, only green light is picked up by the camera.
FILTERING THE LIGHT ON COLLECTION
White light is used to illuminate the scene, but a green filter is physically attached to the camera lens, meaning that only the green component of the image passes through to be recorded.
IMAGE MANIPULATION
A standard photo using white light and a digital camera is taken. Once the digital image has been captured, it is digitally altered to filter out other colors within the image.
The EVA Solution
The EVA System is a digital colposcope that includes its own white light source, which allows the implementation of any of the three methods to create the green light filter. 
Learn more about the other EVA System features.
A central aim in developing the EVA System was to create a user-friendly colposcope that would enable primary care and other non-specialist health workers to perform expert level colposcopy. That required a cost-effective application of the green filter to allow for wide adoption of the EVA System.
A digital filter provides a practical option that enable users to compare the same image under white light and with the green filter applied.
Creating a digital green filter
A digital filter alters the relative value of a color channel’s pixels and creates an image. Many pixels containing a specific level of red, blue and green comprise a digital image. By programing the EVA System application to automatically reduce the level of red and blue appearing within the image when required, the image retains the same core information while giving it a green appearance.
The industry does not have a set standard for the method or shade of green used in a filter. Traditional colposcope green filters range from a bright emerald green, through to various shades of turquoise and even aquamarine. We consulted with a panel of expert colposcopists presenting them a number of different shades of green to see which was the most acceptable.
Read about EVA’s Cloud-based processing of multispectral imaging
Our initial green filter removed all red and blue from the filtered image. This produced a bright green coloration. When we consulted with expert colposcopists, many reported that this was ‘too green’ or ‘too bright.’
We then worked on an adjusted form of the green filter. While we still reduced red to almost nil, blue levels reduced to approximately half, while green remained at full strength. This created a green filter that created a more nuanced image, which has gained wide acceptance within the colposcopy community.
This image demonstrates the difference in initial and current green filters. Showing a color calibration card seen through white light, digitally altered to show the effect of the initial green filter and then through the current EVA green filter.
The green filter represents one of many EVA System features that offer users an advanced women’s health tool. MobileODT remains committed to listening to our customers to give them the highest level mobile colposcope available on the market.
About the author:
David Levitz, PhD, CTO and MobileODT founder
David Levitz, PhD, is the founder and Chief Technology Officer of MobileODT. Dr. Levitz also chairs the “Optics and Biophotonics in Low Resource Settings” conference at SPIE’s annual Photonics West meetings. Dr. Levitz holds a Bachelor’s degree in biomedical engineering/optics from the University of Rochester (Rochester, NY, USA.) He performed post-graduate work at Risø National Laboratory (Roskilde, Denmark), and Lund University (Lund, Sweden) and received his PhD from Oregon Health & Science University (OHSU.) Dr. Levitz was awarded a Whitaker scholarship for a postdoctoral fellowship in Israel.
