Looking to the future: Health benefits of eyepiece-less stereo microscopes

Dick Fielding, Vision Engineering Inspection product specialist, explains why eyepiece-less stereo microscopes are an increasingly healthy option.

Eye relief and exit pupil

If you wear glasses (and most of us do) you may know how difficult it can be to use binoculars. Old models will clatter against your glasses and, even with the rubber eyepieces of more recent models, your glasses soon become smeared from the contact. Much more frustrating is the difficulty in getting a clear unimpeded view. This is because for a comfortable view each eye needs to be at the point in front of the eyepiece where the image is formed. This point is called the ‘exit pupil’ and is typically 4 to 5mm in diameter, or not much bigger than the size of your pupils in daylight. A comfortable viewing position is achieved when the exit pupil completely covers the pupil of the eye and can be held there without difficulty. Problems arise for glasses wearers when their eyes cannot get close enough to the exit pupil. When this happens the image is in focus but the field of view is restricted and it is difficult to keep the exit pupil over (or within) the eye pupil, a slight shift in its position can cause shadowing in the field of view.

Another way of describing this problem is that the exit pupil is too close to the eyepiece lens. This distance, called ‘eye relief’, is around 10 to 15mm for most binoculars. The distance is well named as for glasses wearers it is a relief to use binoculars where it is around 18 or 20mm.

Conventional stereo microscopes and discomfort

Stereo microscopes are typically in the 5x to 100x magnification range and are widely used in medtech component inspection and testing. As with binoculars, they have separate optics for each eyepiece and so each has a different viewpoint of the subject giving the viewer depth perception, or a 3D view. In fact the 3D effect is much stronger with stereo microscopes because the subject is very close and so the difference between the viewpoints is more pronounced.

However, a key problem with conventional stereo microscopes is user discomfort: back fatigue, neck fatigue and eye fatigue. It is caused by poor posture and eye strain as, in general, users need to bend over the instrument to bring their eyes to the eyepieces and must retain a fixed position with very limited head movement. Users are also not able to view the subject directly without moving back from the microscope, looking down and refocusing their eyes. All these are serious drawbacks particularly as tasks often require working for several hours a day, perhaps doing repetitive tasks or tasks requiring close hand-eye coordination, and where accuracy is of critical importance.

The effects on health are significant; surveys suggest 90% of microscope users report visual problems, with headaches reported in over 50% of users. Over 80% of users report neck strain and around 60% musculoskeletal problems in the back .

Glasses wearers have particular difficulty in achieving a comfortable view of the subject yet probably account for the majority of potential users: just under 40% of people in 20 to 40 year age group wear glasses, this increases to 55% for 40 to 50 year olds and to over 80% for 50 to 65 year olds. So the most experienced stereo microscope users are likely to wear prescription glasses. If users needed to wear protective eyewear such as safety goggles, the eyes could not be positioned at the eyepieces and it would be very difficult to use a conventional stereo microscope effectively.

Eyepiece-less design and ergonomics

The main challenge then is to find a way in which the user can sit at the microscope in a comfortable position. The key to achieving this is to increase the eye-relief and the exit pupil diameter of the microscope to allow the viewer to be further from the eyepiece and have some freedom of head movement without causing shadowing. This cannot be achieved purely through optical design considerations. The solution Vision Engineering has developed over many years incorporates a spinning lenticular disc within the optical system and which merges the images formed from each of its 3.5 million micro-lenses. As with conventional stereo microscopes, an image is formed for each eye but crucially the diameter of each (the exit pupil) is several centimetres rather than a few millimetres. The eye relief distance is also substantially increased. No eyepieces are required; the user simply looks into a viewing screen.

The eyepiece-less design enables a comfortable and relaxed viewing posture. Users can work for long periods at the instrument without experiencing discomfort or fatigue, and small movements in eye position no longer cause shadowing. There are other benefits; the increased lateral head movement heightens the depth perception of the 3D image and if the viewer needs to look down at the subject directly, they can do so without shifting position or having to refocus. This greatly eases hand-eye coordination tasks. Because of their contactless design, they are frequently used inside laminar flow cabinets and, most importantly perhaps, glasses wearers enjoy exactly the same comfort and ease-of-use benefits.

Low infection inspection

I began with a binoculars analogy and I was rather dismissive of the annoyance of getting smears on our glasses from contact with the eyepieces. Instead I concentrated on the discomfort for glasses wearers. When translated to stereo microscopes, there are significant health benefits in addressing this discomfort (to say nothing of the productivity benefits). However, it turns out that the most important aspect of eyepiece-less design may be that there are ‘no smears on your glasses’ i.e. that there is no contact between the user’s eyes and the instrument.

The current Coronavirus (COVID-19) crisis is raising awareness of workers’ health in general and particularly  on the need to reduce infection risk in the workplace. Instrumentation is a potential source of contamination. With conventional stereo microscopes, the user’s eyes come into contact with the eyepieces and eyepieces will therefore need to be cleaned thoroughly before and after use. If protective eyewear is a requirement it would be extremely restrictive on the usability of the instrument.

As well as their ergonomic advantages, eyepiece-less stereo microscopes present clear advantages for minimising cross contamination: the viewer’s eyes are not in contact with the instrument, greatly reducing the risk of infection from shared use, and wearing protective eyewear is no restriction to their usability. Hopefully, right now, they are proving a useful tool in containing and defeating this virus.