The original (Mark I) Slide had to be significantly modified and Geoff Pickford was present at the HSE laboratories soon after this occasion and was then able to test several Mark II Slides in a number of laboratories around the world, assisting in the verification that block 5 was an indicator of satisfactory microscope/observer performance (Le Guen, ibid, page 240).
Because of the age and difficult manufacture of the HSE/NPL Mark II Slide, the application of modern nanofabrication techniques, similar to those used for semiconductor fabrication appeared to be promising.
Australian based organisation PhaseSlides commenced early design in 2017 in conjunction with the Australian National Fabrication Facility (ANFF) based at the University of New South Wales (UNSW).
The Pickford Test Slide (Figure 1a) is of rugged stainless steel construction dimensionally similar to a standard microscope slide, and houses four ‘chips’ side by side, labelled Set A, B, C and D.
Each chip (Figure 1b) is 0.5mm thick fused quartz, with a set of 20 etched grooves 1µm wide and 100µm long (Figures 1c and 1d). The grooves range from 80nm deep for Set A, down to 5nm deep for Set D. When not in use, each Slide is housed in a 3D printed container (Figure 1d), and held in place with magnets
The quartz chips are overlaid with a coating of acrylic-based UV activated adhesive which fills the grooves. The adhesive has a refractive index of approximately 1.48 and the quartz has a refractive index of approximately 1.46, so each groove forms a phase object whose visibility depends on groove depth.
On each chip, the set of grooves is surrounded by a series of circular and radial guidelines to help microscopists find the grooves.
The most visible set of phase objects (Set A) is designed to reveal the nature of the grooves to microscopists, and is roughly equivalent to blocks 1 and 2 of the HSE/NPL Mark II Slide.
Set B is equivalent to block 5 of the HSE/NPL Mark II Slide, and all etched grooves should be faintly visible for satisfactory performance.
Set C is equivalent to block 6 of the HSE/NPL Mark II Slide, and some etched grooves may be partly visible, or may be invisible for satisfactory performance.
Set D is equivalent to block 7 of the HSE/NPL Mark II Slide, and no etched grooves should be visible for satisfactory performance.
