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Where d(tot) represents the depth of field, λ is the wavelength of illuminating light, n is the refractive index of the medium (usually air (1.000) or immersion oil (1.515)) between the coverslip and the objective front lens element, and NA equals the objective numerical aperture. The variable e is the smallest distance that can be resolved by a detector that is placed in the image plane of the microscope objective, whose lateral magnification is M. Using this equation, depth of field (d(tot)) and wavelength (λ) must be expressed in similar units. For example, if d(tot) is to be calculated in micrometers, λ must also be formulated in micrometers (700 nanometer red light is entered into the equation as 0.7 micrometers). Notice that the diffraction-limited depth of field (the first term in the equation) shrinks inversely with the square of the numerical aperture, while the lateral limit of resolution is reduced in a manner that is inversely proportional to the first power of the numerical aperture. Thus, the axial resolution and thickness of optical sections that can be attained are affected by the system numerical aperture much more so than is the lateral resolution of the microscope.

Recently we've had some (Japanese) vendors insist on an ISO callout ratherthan the MIL standard for scratch and dig...We're told by our main lens vendor (China) that they're seeing 35-40% (andincreasing) ISO format drawings"gac" wrote in messagenews:416ccabf$1...@127.0.0.1...

Wow! Either I'm buying more optics than I thought, or someone else is using the Internationally Recognized but universally unrecognized standard.As far as the MIL scratch/dig spec, Jean Bennet argued years ago that it was a purely cosmetic spec. If you use it that way, it works well. For instance, if I want laser optics, I spec 20/10, but if I only need commercial grade, I order 80/50. Condensers are 120/80, etc. Just for total confusion, I spec the scratch/dig on my pseudo-ISO prints as 5/80/50!-- Best regards,Steve Eckhardtskeckhardt at mmm dot com

This comes from the paper, "The Scratch Standard Is Only A CosmeticStandard" by Matt Young."Additionally, the startling factor-of-10 change between Revisions Hand L has caused some users to speculate that the primary standardsmay have healed with the passage of time. Certainly the glass arounda scratch cannot flow (the viscosity of glass at room temperature iswell in excess of 10^15 Pa*s), but chemical weathering of the surfacedue to water vapor in the air is a possibility and could cause theprimary standard either to heal or to worsen. There is evidence,however, that most weathering would take place and stabilize within afew hours or days. In any case, a change has to be quite substantialbefore it is apparent to the naked eye and, I think, would be pickedup by the trained inspectors when they make repeated comparisons withsecondary standards. More probably, it seems to me, small scratchesthat are reported to have "disappeared" have been contaminated by thevolatile constituents of their wooden boxes and merely required a gooddegreasing,"Anyway it is the drawing that has changed with time, due to peoplesattempts to define the scratch number as a width. Revision H of thedrawing was the first define the scratch number as the width of thescratch in microns and also that calibration back to the masters isnot essential. However Revision H was quickly surperceeded two yearslater by Revision J which said the scratch width was one tenth thescratch number and all manufacturer generated standards must becalibrated against the master set. In any event nothing in thecertification procedure had changed, nor has it changed to this day. Remember that a certified set of submasters is boxed but nothermetically sealed and therefore it must be resubmitted periodicallyfor recertification - hence my question can you still get comparisonstandards certified against the masters?

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Also, as I read the spec, the scratch number is very subjective and isa comparison to standard samples. However the dig number does relateto the actual width of the dig. The drawing I mentioned in my firstpost describes how to make the set of samples. This drawing has hadmany revisions that state many different things ie, "Scratch numbersdo not denote width of scratch. The numbers indicate that thescratch has the same weight or visual appearance as the masterscratch bearing the same weight number", "Scratch number denoteswidth of scratch in microns", "#10 Scratch = 1um +/- 0.1....", andthe ever popular "Dimensions are for reference only". Anyway as Iunderstand these dimensions on the drawing should be used as a guideto preparing your samples but they must be calibrated against themaster set.Perusing different websites and optics suppliers shows that everyonewants to link the scratch number to a width. Some go with the 10 =10 theory and some go with the 10 = 1. My original question on themaster samples was due to these two different schools of thought iecan anyone do the correct calibration or are companies just measuringthe width of their standard samples and saying they are calibrated? If it is the latter then the scratch criteria has changed to reflectMIL-F-48616 - which allows for the actual measurement of the width ofthe scratch but uses letters instead of numbers eg F-F.

> According to the drawing C7641866 L, released in 1980, the mastersamples have moved from Frankford Arsenal to ARRADCOM. Can you stillget your samples calibrated against the masters as stipulated in Note6 of the drawing?> > If not how do all optics companies keep their scratch setscalibrated?> >

If not how do all optics companies keep their scratch setscalibrated?Thanks.*-----------------------* Posted at: www.GroupSrv.com*-----------------------*

From my understanding the inspection is, as you mentioned, with a 40Wlight bulb etc. Also, as I read the spec, the scratch number is very subjective and isa comparison to standard samples. However the dig number does relateto the actual width of the dig. The drawing I mentioned in my firstpost describes how to make the set of samples. This drawing has hadmany revisions that state many different things ie, "Scratch numbersdo not denote width of scratch. The numbers indicate that thescratch has the same weight or visual appearance as the masterscratch bearing the same weight number", "Scratch number denoteswidth of scratch in microns", "#10 Scratch = 1um +/- 0.1....", andthe ever popular "Dimensions are for reference only". Anyway as Iunderstand these dimensions on the drawing should be used as a guideto preparing your samples but they must be calibrated against themaster set.Perusing different websites and optics suppliers shows that everyonewants to link the scratch number to a width. Some go with the 10 =10 theory and some go with the 10 = 1. My original question on themaster samples was due to these two different schools of thought iecan anyone do the correct calibration or are companies just measuringthe width of their standard samples and saying they are calibrated? If it is the latter then the scratch criteria has changed to reflectMIL-F-48616 - which allows for the actual measurement of the width ofthe scratch but uses letters instead of numbers eg F-F.

At high numerical apertures of the microscope, depth of field is determined primarily by wave optics, while at lower numerical apertures, the geometrical optical circle of confusion dominates the phenomenon. Using a variety of different criteria for determining when the image becomes unacceptably sharp, several authors have proposed different formulas to describe the depth of field in a microscope. The total depth of field is given by the sum of the wave and geometrical optical depths of fields as:

I have a question regarding the MIL spec that has become the industrystandard for specifying scratch-dig.I realise that the scratch number doesn't actually define the width ofscratches (like many people beleive) but references the samples usedfor comparison. According to the drawing C7641866 L, released in 1980, the mastersamples have moved from Frankford Arsenal to ARRADCOM. Can you stillget your samples calibrated against the masters as stipulated in Note6 of the drawing?If not how do all optics companies keep their scratch setscalibrated?Thanks.*-----------------------* Posted at: www.GroupSrv.com*-----------------------*

According to the drawing C7641866 L, released in 1980, the mastersamples have moved from Frankford Arsenal to ARRADCOM. Can you stillget your samples calibrated against the masters as stipulated in Note6 of the drawing?If not how do all optics companies keep their scratch setscalibrated?Thanks.*-----------------------* Posted at: www.GroupSrv.com*-----------------------*

Information from OEOSC (who are developing a new standard due outearly next year) indicate that the old scratch dig comparisonstandard is carrying over to the new standard.

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Perusing different websites and optics suppliers shows that everyonewants to link the scratch number to a width. Some go with the 10 =10 theory and some go with the 10 = 1. My original question on themaster samples was due to these two different schools of thought iecan anyone do the correct calibration or are companies just measuringthe width of their standard samples and saying they are calibrated? If it is the latter then the scratch criteria has changed to reflectMIL-F-48616 - which allows for the actual measurement of the width ofthe scratch but uses letters instead of numbers eg F-F.

I realise that the scratch number doesn't actually define the width ofscratches (like many people beleive) but references the samples usedfor comparison. According to the drawing C7641866 L, released in 1980, the mastersamples have moved from Frankford Arsenal to ARRADCOM. Can you stillget your samples calibrated against the masters as stipulated in Note6 of the drawing?If not how do all optics companies keep their scratch setscalibrated?Thanks.*-----------------------* Posted at: www.GroupSrv.com*-----------------------*

Information from OEOSC (who are developing a new standard due outearly next year) indicate that the old scratch dig comparisonstandard is carrying over to the new standard.

Axial resolution, like horizontal resolution, is determined only by the numerical aperture of the objective (Figure 2), with the eyepiece merely magnifying the details resolved and projected into the intermediate image plane. Just as in classical photography, depth of field is determined by the distance from the nearest object plane in focus to that of the farthest plane also simultaneously in focus. In microscopy depth of field is very short and usually measured in units of microns. The term depth of focus, which refers to image space, is often used interchangeably with depth of field, which refers to object space.

Depth of focus varies with numerical aperture and magnification of the objective, and under some conditions, high numerical aperture systems (usually with higher magnification power) have deeper focus depths than do those systems of low numerical aperture, even though the depth of field is less (see Table 1). This is particularly important in photomicrography because the film emulsion or digital camera sensor must be exposed or illuminated in a plane that falls within the focus region. Small errors made to focus at high magnification are not as critical as those made with very low magnification objectives. Table 1 presents calculated variations in the depth of field and image depth in the intermediate image plane in a series of objectives with increasing numerical aperture and magnification.

These values for the depth of field, and the distribution of intensities in the three-dimensional diffraction pattern, are calculated for incoherently illuminated (or emitting) point sources where the numerical aperture of the condenser is greater than or equal to that of the objective. In general, the depth of field increases, up to a factor of 2, as the coherence of illumination increases (as the condenser numerical aperture approaches zero). However, the three-dimensional point spread function (PSF) with partially coherent illumination can depart in complex ways from that so far discussed when the aperture function is not uniform. In a number of phase-based, contrast-generating modes of microscopy, the depth of field may turn out to be unexpectedly shallower than that predicted from the equation above and may yield extremely thin optical sections.

"Additionally, the startling factor-of-10 change between Revisions Hand L has caused some users to speculate that the primary standardsmay have healed with the passage of time. Certainly the glass arounda scratch cannot flow (the viscosity of glass at room temperature iswell in excess of 10^15 Pa*s), but chemical weathering of the surfacedue to water vapor in the air is a possibility and could cause theprimary standard either to heal or to worsen. There is evidence,however, that most weathering would take place and stabilize within afew hours or days. In any case, a change has to be quite substantialbefore it is apparent to the naked eye and, I think, would be pickedup by the trained inspectors when they make repeated comparisons withsecondary standards. More probably, it seems to me, small scratchesthat are reported to have "disappeared" have been contaminated by thevolatile constituents of their wooden boxes and merely required a gooddegreasing,"Anyway it is the drawing that has changed with time, due to peoplesattempts to define the scratch number as a width. Revision H of thedrawing was the first define the scratch number as the width of thescratch in microns and also that calibration back to the masters isnot essential. However Revision H was quickly surperceeded two yearslater by Revision J which said the scratch width was one tenth thescratch number and all manufacturer generated standards must becalibrated against the master set. In any event nothing in thecertification procedure had changed, nor has it changed to this day. Remember that a certified set of submasters is boxed but nothermetically sealed and therefore it must be resubmitted periodicallyfor recertification - hence my question can you still get comparisonstandards certified against the masters?

Just for total confusion, I spec the scratch/dig on my pseudo-ISO prints as 5/80/50!-- Best regards,Steve Eckhardtskeckhardt at mmm dot com

When considering resolution in optical microscopy, a majority of the emphasis is placed on point-to-point lateral resolution in the plane perpendicular to the optical axis (Figure 1). Another important aspect to resolution is the axial (or longitudinal) resolving power of an objective, which is measured parallel to the optical axis and is most often referred to as depth of field.

The human eye can normally accommodate from infinity to about 25 centimeters, so that the depth of field can considerably greater than that given by the equation above when one observes the microscope image through the eyepieces. On the other hand, a video sensor or photographic emulsion lies in a thin fixed plane so that the depth of field and axial resolution using those sensors are given by the parameters in the equation. In these cases, the axial resolution is defined by convention as one-quarter of the distance between the first minima, above and below focus, along the axis of the three-dimensional diffraction image produced by the objective.

In digital and video microscopy, the shallow focal plane in the target of the camera tube or CCD, the high contrast achievable at high objective and condenser numerical apertures, and the high magnification of the image displayed on the monitor all contribute to reducing the depth of field. Thus, with video, we can obtain very sharp and thin optical sections, and can define the focal level of a thin specimen with very high precision.

As far as the MIL scratch/dig spec, Jean Bennet argued years ago that it was a purely cosmetic spec. If you use it that way, it works well. For instance, if I want laser optics, I spec 20/10, but if I only need commercial grade, I order 80/50. Condensers are 120/80, etc. Just for total confusion, I spec the scratch/dig on my pseudo-ISO prints as 5/80/50!-- Best regards,Steve Eckhardtskeckhardt at mmm dot com

Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.

Information from OEOSC (who are developing a new standard due outearly next year) indicate that the old scratch dig comparisonstandard is carrying over to the new standard.

Anyway it is the drawing that has changed with time, due to peoplesattempts to define the scratch number as a width. Revision H of thedrawing was the first define the scratch number as the width of thescratch in microns and also that calibration back to the masters isnot essential. However Revision H was quickly surperceeded two yearslater by Revision J which said the scratch width was one tenth thescratch number and all manufacturer generated standards must becalibrated against the master set. In any event nothing in thecertification procedure had changed, nor has it changed to this day. Remember that a certified set of submasters is boxed but nothermetically sealed and therefore it must be resubmitted periodicallyfor recertification - hence my question can you still get comparisonstandards certified against the masters?

This interchange of nomenclature can lead to confusion, especially when the terms are both used specifically to denote depth of field in microscope objectives. The geometric image plane might be expected to represent an infinitely thin section of the specimen, but even in the absence of aberrations, each image point is spread into a diffraction figure that extends above and below this plane. The Airy disk, a basic unit of the diffraction pattern produced by the microscope objective, represents a section through the center of the intermediate image plane. This increases the effective in-focus depth of the Z-axis Airy disk intensity profile that passes through slightly different specimen planes.

I would contend that this is the industry standard. It is true thatthis MIL spec keeps appearing but in my experience of manufacture ofhigh end optics this does not apply. I'm a little rusty about theactual methods used to apply this spec but doesn't it state that itshould be a visual comparison by eye to the scratch/dig standard undera 40 watt light in a darkened booth viewed at 45 degrees.If I was to inspect and pass optical surfaces in that manner I'd passeverything..... Oh wait what am I fighting it for !!??Taff

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