I have not always been a medical photographer, indeed my first job was stacking freezer cabinets full of frozen food. I swiftly moved on to working in the Department of Transport and then into the insurance industry working in the city. It was after being made redundant three times in a row as the insurance industry shrank due to new technology, that I contemplated my outside interest as a future career.
My first truly scientific image was taken as a student whilst undertaking my HNC in Medical and Technical Photography at the Berkshire College of Art and Design. Prior to that I had taken many photographs, but none could really be called scientific, certainly my first medical photograph was to come quite a bit later and was far from a pretty image. Therefore, this is not about a medical photographic procedure but about the first image I took that started me on the road to being a medical photographer.
At the time in question, my tutor was the quiet but hugely knowledgeable Eric Pascoe, I have no idea if he is still alive and if he is I hope he doesn’t mind me mentioning him. What Eric did not know about analogue photography in practice was not needed, digital was in its infancy at the time and the quality was dire. I have always loved mechanical devices of one sort or another, so when we were introduced to an optical bench, mirrors, high intensity point source lighting and a razor blade! I was intrigued. Eric in his softly spoken voice announced we were going to photograph air, the stuff all around us that we breathe, take for granted and cannot see…or can we?
We had just been introduced to Schlieren photography, the word originates from the German plural meaning streaks or striae, in effect a schlieren image is a form of shadowgraph. The principle goes back to 1850’s when telescope mirrors were being perfected to provide greater image homogeneity, whereupon the French physicist Leon Foucault described a simple method to show the defects in a concave mirror finish. The process involved a point source light being directed through a slit to the mirror in question and the image formed then viewed by the naked eye via a knife edge at the mirror’s primary focus. The imperfections in the mirror cause variations in the point of focus of the reflected light which in turn results in it either being blocked or passing by the knife edge forming a phase image when viewed by the naked eye. A perfect mirror shows a uniform and evenly lit image, variations in the image density seen are the result if grinding and polishing of the mirror did not form a perfect parabola.
August Toepler revised Foucault’s knife edge test for telescope mirrors by introducing lenses enabling him to examine the fault of varying density in optical glass. However there are limitations to this solution imposed by the lenses residual chromatic aberrations. To overcome this problem a further refinement of two highly accurate spherical mirrors can be used providing a wider diameter collimated beam of light for the subject to sit in.
This setup for a double concave schlieren system is simple but it needs to be on a very rigid and vibration free optical bench to provide the extremely fine adjustment required to gain a high-quality image. The front silvered mirrors used, depending on their size can be heavy, expensive, and exceedingly easy to damage.
Schlieren photography can be adapted to high-speed cinematic images and NASA have produced images of supersonic flight shockwaves using the sun as the collimated light beam. A simple trawl of the internet will uncover visually stunning examples and on topic clinical images, thanks to Covid, showing air dispersion due to coughing with and without face masks.
The image below was taken using a double concave mirror system, the light source was a high intensity fibre optic projected through a pinhole and a very fine vertical RGB colour grating was used instead of a knife edge. The optical defects of the well-used mirrors are plain to see in the surrounding background, whereas the overall image is soft due to vibration of the optical system from a nearby road! Yes it’s that sensitive. The three colour grating allows the difference in air density to be imaged by utilising the variation in refraction of the collimated light rays and therefore the point of image focus. In essence the refracted light rays will either pass through one of the filter bands or pass completely by it as shown in the diagram. Naturally if they hit a solid object the result is a black shadow, the tacking iron in this case.
The image I took is not great, it’s low quality, it’s not exciting either, it’s just a hot tacking iron which was all I had handy at the time, and still have, tucked away in a drawer. It will not win any awards compared to other examples, but it was the first image taken many years ago at the start of my career in the medical, technical, and scientific world of imaging and one that has kept me interested in the medium ever since.
So, to sum up, did we photograph air…no, just the difference in the refractive index due to localised density variation caused by a hot tacking iron, it was good fun though.