COMPARISON OF HSE/NPL MARK II AND PICKFORD TEST SLIDES
The Pickford Phase Contrast Test Slide has been designed to be an alternative and eventual replacement for the HSE/NPL Mark II Phase Contrast Test Slide.
The HSE/NPL Mark II Test Slide was designed 40 years ago using technology of the day. It has been an invaluable tool in standardising microscopes and analyst performance globally, for laboratories determining airborne asbestos fibre concentrations.
The Pickford Phase Contrast Test Slide has been designed using state of art, semiconductor, nanofabrication processes, and is different in a number of ways, but uses the same principles as applying to the HSE/NPL Mark II Test Slide. The HSE/NPL Mark II Slide has 7 blocks of ridges (acting as phase objects) which in general has the most visible of the blocks (1, 2, 3, and 4) designed to show the observer what they look like, and as a guide to be able to locate other blocks that are difficult to detect. Block 5 should be faint but just visible in the entirety of the ridges; block 6 may or may not be partly visible; and block 7 should not be visible at all. It should be noted that for some HSE/NPL Mark II Slides examined, block 7 ridges are completely immersed in ‘noise’ thus being effectively invisible when using any form of light microscopy (possibly except dark field). Even when fully formed, block 7 ridges are near to or below the detection limit of opaque objects, and even less visible when using phase contrast microscopy.
In comparison, the Pickford Test Slide has 4 Sets of phase objects in the form of etched grooves, each Set on an individual quartz chip 8 mm high, 4 mm wide and 0.5 mm thick. Current production has all Sets on the one large chip. The depths of the grooves are different for each Set ranging from 90 nm down to 5 nm which is the limit of nanofabrication technology. The most visible Set (Set A) is designed to reveal the nature of the phase objects to analysts – actually 20 etched vertical grooves, 1 µm wide, spaced at 5 µm pitch, and 100 µm long. Sets B, C and D are equivalent to blocks 5, 6 and 7 of the HSE/NPL Mark II Slide.
The HSE/NPL Mark II Slide proves quite difficult to use because it is hard to locate blocks 5, 6 and 7, and to remember which is which, whereas the use of radial and concentric circle guidelines on each Chip of the Pickford Test Slide makes it easy for any observer to very quickly locate and identify the grooves with certainty.
In summary, the new Test Slide is of rugged stainless steel construction; easier to use; based on pure phase objects designed to test an analyst and microscope’s limit of visibility in a similar fashion to viewing very fine asbestos fibres; and is believed to comprise more stable and better tested optical components when compared with the HSE/NPL Mark II Slide.
