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bright field dark field microscopy

 

1
Released 4/20/2016
Bright Field vs. Dark Field in Vision
One of the more diffi cult concepts in
machine vision lighting is recognizing when
it is advantageous to use dark fi eld lighting
over its more commonly applied bright
fi eld counterpart. Both techniques have
advantages and disadvantages; whereas
bright fi eld lighting has a wider application
envelope for most samples, dark fi eld
lighting has a more limited set of conditions
necessary for its successful application. We
will concentrate on a comparison – contrast
between bright fi eld (BF) and dark fi eld (DF)
lighting using common vision applications.
As used in vision inspection, bright fi eld lighting
is defi ned as lighting that is primarily incident
on the fi eld of view from a source oriented at
greater than 45 degrees relative to the sample
surface. Bright fi eld can be further divided into 2
sub-techniques, and solid angle – a measure of
the amount of relative area from which the light
is sourced – is an effective differentiator. Bar,
spot and ring lights, or any light at a signifi cant
working distance (WD), have a relatively low
solid angle, and are therefore termed partial
or directional BF point sources (Fig. 1a).
Conversely, high solid angle hemispherical
or planar lights, such as diffuse domes and
cylinders (Fig. 1b), or axial diffuse (Fig. 1c) and
“flat” arrays (Fig. 1d), respectively, are termed full
bright fi eld sources.
Consider also, for a full bright fi eld light to be
effective, hence subtending a large solid angle,
it must be placed relatively close to the sample.
Fig. 1b
Diffuse Dome
Light Function
Fig. 1c
Axial Diffuse Light
Function
Fig. 1a
Partial Bright
Field Function
Fig. 2a
Dark Field
Fig. 2b
Low Angle Dark
Field
Fig. 1d
Flat Diffuse
Illuminator
2
Released 4/20/2016
Bright Field vs. Dark Field in Vision
Thus it follows that as a light’s WD increases, its solid angle decreases,
rendering the effect of a full BF light more typical of a partial BF light.
Why is this dichotomy important? Simply because each combination of
light presentation, geometry, WD and solid angle has its own advantages
depending on the sample characteristics, inspection features of interest,
and sample access considerations, to name just a few.
If a bright fi eld is characterized as the result of high angle incident light
producing a “bright” fi eld of view, then we can correctly conclude that dark
fi eld lighting can be said to generate a primarily “dark” fi eld of view, at low
angles of incidence (Fig. 2a). How can this be? How can light produce
a “dark” fi eld? Dark fi eld lighting was fi rst used in microscopy and was
defi ned by circular light incident on a sample surface at 45 degrees. As
commonly used in machine vision today, we also see very low angle DF
with incident light as low as 10-15 degrees from the sample surface (Fig.
2b), as well as from a single direction, not just from circular sources.
Figures 3a & 3b illustrate the results of how BF and DF light responds
differently on a mirrored surface.
To fully understand how dark fi eld light is produced and used, it
is important to remember a simple physical property of incident
light: Fig. 3b Dark Field Set Up and Image scratch the angle of
reflection is equal to the angle of incidence. Further, as a corollary,
it is the actual detail of the surface features that determines how
and where light reflects. If we examine the angle of incidence and
similarly project what the angle of reflection of the light function
diagrams in Figs. 3a & 3b, would be, we can start to understand
how dark fi eld is produced.
For example, with the BF ring light, if we project the amount of
light reflected from the mirror that actually returns back into
the lens, we see that it is quite large; in fact, most of the light is
reflected into the camera. This effect produces the image we see
in Fig. 3a, typically referred to as a specular hot spot. Comparing
the projected amount of light from the low angle DF ring light
Fig. 3a
Bright Field Set
Up and Image
Camera
BF Ring Light
Mirror
Fig. 3b
Dark Field Set Up
and Image
Camera
DF Ring Light
Mirror
Scratch
3
Released 4/20/2016
Bright Field vs. Dark Field in Vision
in Fig. 3b, we see clearly that most of the light reflects away from the
camera, and thus is not collected, hence we see a “dark fi eld”. Naturally
this begs the question – how is this fact useful?
Consider the above-mentioned corollary: It is the individual surface
details that reflect differently from the overall mirrored surface, and
some of the light reflected off these surface imperfections reaches
the camera (See Fig 4). In this fashion, we can effectively inspect the
surface of a mirror for scratches.
Figures 5a and 5b illustrate another
example of the robustness of dark
fi eld vs. bright fi eld lighting for some
common inspections. The image
depicted in Fig. 5a was captured
with a standard coaxial BF ring light,
whereas the image in Fig. 5b was
generated by a linear bar (Fig. 5c –
AL4424-660 BALA) oriented from the
side in classic dark fi eld geometry. We can see that either image is likely suitable
as-is, but consider if the next sample had considerable dark staining: The DF image
likely would not change, whereas the stain might be plainly visible in the bright fi eld
image, and thus more likely to affect the inspection results.
Does it necessarily follow that all dark fi eld lights are applied at very low angles of
incidence, to produce a completely dark fi eld, except for surface abnormalities? No.
In the following example, we see that by using a light off axis near 45 degrees, we
can take advantage of the dark fi eld effect, thus erasing a specular glare problem In
the following example, we see that by using a light off axis near 45 degrees, we can
take advantage of the dark fi eld effect, thus erasing a specular glare problem.
The series of images in Fig. 6 illustrates the effect of applying both ring and bar lights at an angle that allows
the majority of the light to reflect away from the camera, thus eliminating specular glare, yet still allowing
enough captured fi eld lighting to view the surface label and details. The image in Fig. 6a shows specular
reflection of a co-axial bright fi eld light. Compare this image with that in Fig. 6b where the same light was
Fig. 4
Ray Function
Diagram Dark Field
Fig. 5a
Dot Peen in
Bright Field
Fig. 5b
Dot Peen in
Dark Field
Fig. 5c
BALA Function
Diagram
4
Released 4/20/2016
Bright Field vs. Dark Field in Vision
moved off-axis to produce an acceptable result for inspection. Similarly, a
high intensity array light (Figs. 6c & d) may be used mounted transversely
to the bottle length from a greater WD to produce the same acceptable
inspection result if part access is limited.
We have compared the application and results of bright and dark fi eld
lighting techniques, but there are some usage criteria to consider for each.
Directional or partial BF lights are the most versatile, from a positioning
stand point, so long as they don’t produce specular glare; i.e. – try imaging
the surface of a ball-bearing with a ring light. Full BF lights, particularly the
diffuse dome and cylinder varieties generally
need to be in close proximity to the sample,
and also may need to be selected with
specifi c lenses in mind to avoid vignetting
issues, and there is always the possibility
that these lights may block part access,
particularly in a vision guided robotics
implementation. Dark fi eld lights, particularly
the circular varieties also must be placed very
close to the part, and suffer similar problems as full BF lights. Assuming
circular DF is not necessary, bar lights, of suffi cient power, can be placed
in a dark fi eld orientation from a longer WD, alleviating some part access
issues. Almost any light, except for diffuse area lights and back lights can
be used in a dark fi eld orientation, namely 45 degrees or less to the sample
surface.
Fig. 6a
Coaxial BF Ring
Light
Fig. 6b BF Ring
Light at Low
Angle
Fig. 6c Linear
Array Light
Fig. 6d
AL143

 

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