Using colors that are composed predominantly of mid spectral components for the light and spectral extremes (blue and red wavelengths) for the dark

Yes, a darkfield microscope can be equipped with a camera for recording images observed with the contrast method. It can also be equipped with other accessories. For more information, contact your local Leica representative.

Hues are perceived differently by users with color vision deficiencies (both congenital and acquired) resulting in different colors and relative luminance contrasts than for normally sighted users. Because of this, effective contrast and readability are different for this population. However, color deficiencies are so diverse that prescribing effective general use color pairs (for contrast) based on quantitative data is not feasible. Requiring good luminance contrast accommodates this by requiring contrast that is independent of color perception. Fortunately, most of the luminance contribution is from the mid and long wave receptors which largely overlap in their spectral responses. The result is that effective luminance contrast can generally be computed without regard to specific color deficiency, except for the use of predominantly long wavelength colors against darker colors (generally appearing black) for those who have protanopia. (We provide an advisory technique on avoiding red on black for that reason). For more information see [ARDITI-KNOBLAUCH] [ARDITI-KNOBLAUCH-1996] [ARDITI].

Note: Some people with cognitive disabilities require color combinations or hues that have low contrast, and therefore we allow and encourage authors to provide mechanisms to adjust the foreground and background colors of the content. Some of the combinations that could be chosen may have contrast levels that will be lower than those found in the Success Criteria. This is not a violation of this Success Criteria provided there is a mechanism that will return to the default values set out in the Success Criteria.

F83: Failure of Success Criterion 1.4.3 and 1.4.6 due to using background images that do not provide sufficient contrast with foreground text (or images of text)

Note 1: When evaluating this success criterion, the font size in points should be obtained from the user agent or calculated on font metrics in the way that user agents do. Point sizes are based on the CSS pt size as defined in CSS3 Values. The ratio between sizes in points and CSS pixels is 1pt = 1.333px, therefore 14pt and 18pt are equivalent to approximately 18.5px and 24px.

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Darkfield is an optical contrast technique for microscopy which makes unstained structures in the cells of biological specimens visible. Cell structures that appear transparent with brightfield illumination can be viewed with better contrast and detail using darkfield. Additionally, non-uniform features of transparent material or on the surface of opaque material samples can be more easily observed with darkfield compared to brightfield. Structures in the cell or features of the material scatter the light that interacts with them, while uniform areas allow the light to pass without scattering. When using darkfield microscopy, the cell structures or material features appear brighter and the uniform cell or material background look darker.

Note 2: See also Understanding Success Criterion 2.4.7 Focus Visible for techniques for indicating keyboard focus.

Note 2: Because different image editing applications default to different pixel densities (e.g. 72 PPI or 96 PPI), specifying point sizes for fonts from within an image editing application can be unreliable when it comes to presenting text at a specific size. When creating images of large-scale text, authors should ensure that the text in the resulting image is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the default size for body text as rendered by the browser.

Note 5: When there is a border around the letter, the border can add contrast and would be used in calculating the contrast between the letter and its background. A narrow border around the letter would be used as the letter. A wide border around the letter that fills in the inner details of the letters acts as a halo and would be considered background.

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil ...

Instructions: Select the situation below that matches your content. Each situation includes techniques or combinations of techniques that are known and documented to be sufficient for that situation.

G174: Providing a control with a sufficient contrast ratio that allows users to switch to a presentation that uses sufficient contrast

Using a light pastel background rather than a white background behind black text to create sufficient but not extreme contrast (future link)

This Web page is part of Understanding WCAG 2.0: A guide to understanding and implementing WCAG 2.0 (see the latest version of this document). The entire document is also available as a single HTML file. See the The WCAG 2.0 Documents for an explanation of how this document fits in with other Web Content Accessibility Guidelines (WCAG) 2.0 documents. To send public comments, please follow the Instructions for Commenting on WCAG 2.0 Documents.

2017424 — The diffuse light is observed when the projected shadow of the subject has blurred edges: this is called soft shadow. The contrast is also less ...

Dark field microscopy

Note 3: It is sometimes helpful for authors to not specify colors for certain sections of a page in order to help users who need to view content with specific color combinations to view the content in their preferred color scheme. Refer to Understanding Success Criterion 1.4.5 Images of Text for more information.

The intent of this Success Criterion is to provide enough contrast between text and its background so that it can be read by people with moderately low vision (who do not use contrast-enhancing assistive technology). For people without color deficiencies, hue and saturation have minimal or no effect on legibility as assessed by reading performance (Knoblauch et al., 1991). Color deficiencies can affect luminance contrast somewhat. Therefore, in the recommendation, the contrast is calculated in such a way that color is not a key factor so that people who have a color vision deficit will also have adequate contrast between the text and the background.

1.4.3 Contrast (Minimum): The visual presentation of text and images of text has a contrast ratio of at least 4.5:1, except for the following: (Level AA)

The contrast ratio of 4.5:1 was chosen for level AA because it compensated for the loss in contrast sensitivity usually experienced by users with vision loss equivalent to approximately 20/40 vision. (20/40 calculates to approximately 4.5:1.) 20/40 is commonly reported as typical visual acuity of elders at roughly age 80. [GITTINGS-FOZARD]

For forensic applications concerning the evidentiary investigation of paints, pigments, textiles, fibers, and human tissues, Leica microscopes offering darkfield are very useful solutions.

Although this Success Criterion only applies to text, similar issues occur for content presented in charts, graphs, diagrams, and other non-text-based information. Content presented in this manner should also have good contrast to ensure that more users can access the information.

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Not all products or services are approved or offered in every market, and approved labelling and instructions may vary between countries. Please contact your local representative for further information.

Bright field dark field

Note: Calculations in [ISO-9241-3] and [ANSI-HFES-100-1988] are for body text. A relaxed contrast ratio is provided for text that is much larger.

Note 2: Because different image editing applications default to different pixel densities (e.g. 72 PPI or 96 PPI), specifying point sizes for fonts from within an image editing application can be unreliable when it comes to presenting text at a specific size. When creating images of large-scale text, authors should ensure that the text in the resulting image is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the default size for body text. For example, for a 72 PPI image, an author would need to use approximately 19 pt and 24 pt font sizes in order to successfully present images of large-scale text to a user.

Note 1: Multiple user interface components may be implemented as a single programmatic element. Components here is not tied to programming techniques, but rather to what the user perceives as separate controls.

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F24: Failure of Success Criterion 1.4.3, 1.4.6 and 1.4.8 due to specifying foreground colors without specifying background colors or vice versa

A darkfield microscope is similar to a conventional brightfield microscope, except it uses an annular aperture in front of the light source. The light from darkfield illumination impinges onto the specimen or sample at a high angle of incidence, either transmits through the sample or reflects off its surface, then passes through the objective lens, and finally goes through the eyepieces or reaches the camera sensor. Darkfield illumination causes uniform areas of transparent samples or flat surfaces of opaque samples to appear dark, as the vast majority of the light at the high incident angle misses the objective. Normally there are features in transparent samples or on the surface of opaque samples which scatter light. For this reason, darkfield microscopy images show a dark background with brighter areas corresponding to these features, because the light they scatter enters into the objective.

Note 6: WCAG conformance should be evaluated for color pairs specified in the content that an author would expect to appear adjacent in typical presentation. Authors need not consider unusual presentations, such as color changes made by the user agent, except where caused by authors' code.

Text that is decorative and conveys no information is excluded. For example, if random words are used to create a background and the words could be rearranged or substituted without changing meaning, then it would be decorative and would not need to meet this criterion.

Example: An applet has a "control" that can be used to move through content by line or page or random access. Since each of these would need to have a name and be settable independently, they would each be a "user interface component."

This requirement applies to situations in which images of text were intended to be understood as text. Incidental text, such as in photographs that happen to include a street sign, are not included. Nor is text that for some reason is designed to be invisible to all viewers. Stylized text, such as in corporate logos, should be treated in terms of its function on the page, which may or may not warrant including the content in the text alternative. Corporate visual guidelines beyond logo and logotype are not included in the exception.

The minimum contrast success criterion (1.4.3) applies to text in the page, including placeholder text and text that is shown when a pointer is hovering over an object or when an object has keyboard focus. If any of these are used in a page, the text needs to provide sufficient contrast.

The knowledge portal of Leica Microsystems offers scientific research and teaching material on the subjects of microscopy. The content is designed to support beginners, experienced practitioners and scientists alike in their everyday work and experiments.

Although not required for conformance, the following additional techniques should be considered in order to make content more accessible. Not all techniques can be used or would be effective in all situations.

Each numbered item in this section represents a technique or combination of techniques that the WCAG Working Group deems sufficient for meeting this Success Criterion. However, it is not necessary to use these particular techniques. For information on using other techniques, see Understanding Techniques for WCAG Success Criteria, particularly the "Other Techniques" section.

Note 3: For the purpose of Success Criteria 1.4.3 and 1.4.6, contrast is measured with respect to the specified background over which the text is rendered in normal usage. If no background color is specified, then white is assumed.

Note 1: When evaluating this success criterion, the font size in points should be obtained from the user agent or calculated on font metrics in the way that user agents do. Point sizes are based on the CSS pt size as defined in CSS3 Values. The ratio between sizes in points and CSS pixels is 1pt = 1.333px, therefore 14pt and 18pt are equivalent to approximately 18.5px and 24px.

When working with a microscope, the most commonly used contrast method is brightfield. However, brightfield usually only provides a low-contrast image of many transparent biological specimens. It can also be the case for many transparent or reflective opaque material samples. In such low contrast images, few details are distinguished. The contrast of brightfield microscopy for biological specimens can be enhanced using selective stains, but they are often toxic to live cells. A darkfield microscope offers a way to view the structures of many types of biological specimens in greater contrast without the need of stains. Darkfield microscopy can also increase contrast when imaging material samples. The darkfield contrast method exploits diffraction or scattering of light from structures of a biological specimen or non-uniform features of a material sample.

Very often biological specimens and tissues, whether fixed or live, are observed with a darkfield microscope. Also, various material and geological samples can be observed with darkfield. For examples, refer to the articles: "Work More Efficiently in Developmental Biology With Stereo Microscopy: Zebrafish, Medaka, and Xenopus" & "How to Adapt Grain Size Analysis of Metallic Alloys to Your Needs"

Text that is larger and has wider character strokes is easier to read at lower contrast. The contrast requirement for larger text is therefore lower. This allows authors to use a wider range of color choices for large text, which is helpful for design of pages, particularly titles. 18 point text or 14 point bold text is judged to be large enough to require a lower contrast ratio. (See The American Printing House for the Blind Guidelines for Large Printing and The Library of Congress Guidelines for Large Print under Resources). "18 point" and "bold" can both have different meanings in different fonts but, except for very thin or unusual fonts, they should be sufficient. Since there are so many different fonts, the general measures are used and a note regarding fancy or thin fonts is included.

Incidental: Text or images of text that are part of an inactive user interface component, that are pure decoration, that are not visible to anyone, or that are part of a picture that contains significant other visual content, have no contrast requirement.

Note: Images of text do not scale as well as text because they tend to pixelate. It is also harder to change foreground and background contrast and color combinations for images of text, which is necessary for some users. Therefore, we suggest using text wherever possible, and when not, consider supplying an image of higher resolution.

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G156: Using a technology that has commonly-available user agents that can change the foreground and background of blocks of text

The ANSI/HFS 100-1988 standard calls for the contribution from ambient light to be included in the calculation of L1 and L2. The .05 value used is based on Typical Viewing Flare from [IEC-4WD] and the [sRGB] paper by M. Stokes et al.

People with low vision often have difficulty reading text that does not contrast with its background. This can be exacerbated if the person has a color vision deficiency that lowers the contrast even further. Providing a minimum luminance contrast ratio between the text and its background can make the text more readable even if the person does not see the full range of colors. It also works for the rare individuals who see no color.

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Note 2: Because authors do not have control over user settings as to how text is rendered (for example font smoothing or anti-aliasing), the contrast ratio for text can be evaluated with anti-aliasing turned off.

G148: Not specifying background color, not specifying text color, and not using technology features that change those defaults

Tool to convert images based on color loss so that contrast is restored as luminance contrast when there was only color contrast (that was lost due to color deficiency)

Leica microscopes capable of darkfield make a difference for the study of transparent and opaque materials, minerals, crystals, and polymers.

G148: Not specifying background color, not specifying text color, and not using technology features that change those defaults

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Note 1: Refer to related resources for a list of tools that utilize the contrast ratio to analyze the contrast of Web content.

Text that is larger and has wider character strokes is easier to read at lower contrast. The contrast requirement for larger text is therefore lower. This allows authors to use a wider range of color choices for large text, which is helpful for design of pages, particularly titles. 18 point text or 14 point bold text is judged to be large enough to require a lower contrast ratio. (See The American Printing House for the Blind Guidelines for Large Printing and The Library of Congress Guidelines for Large Print under Resources). "18 point" and "bold" can both have different meanings in different fonts but, except for very thin or unusual fonts, they should be sufficient. Since there are so many different fonts, the general measures are used and a note regarding fancy or thin fonts is included.

The rationale is based on a) adoption of the 3:1 contrast ratio for minimum acceptable contrast for normal observers, in the ANSI standard, and b) the empirical finding that in the population, visual acuity of 20/40 is associated with a contrast sensitivity loss of roughly 1.5 [ARDITI-FAYE]. A user with 20/40 would thus require a contrast ratio of 3 * 1.5 = 4.5 to 1. Following analogous empirical findings and the same logic, the user with 20/80 visual acuity would require contrast of about 7:1.

sequence of characters that can be programmatically determined, where the sequence is expressing something in human language

A darkfield microscope is a compound microscope which uses an aperture in the shape of an annulus that is placed between the light source and condenser lens.

In this provision there is an exception that reads "that are part of a picture that contains significant other visual content,". This exception is intended to separate pictures that have text in them from images of text that are done to replace text in order to get a particular look.

G174: Providing a control with a sufficient contrast ratio that allows users to switch to a presentation that uses sufficient contrast

with at least 18 point or 14 point bold or font size that would yield equivalent size for Chinese, Japanese and Korean (CJK) fonts

Note 3: The actual size of the character that a user sees is dependent both on the author-defined size and the user's display or user-agent settings. For many mainstream body text fonts, 14 and 18 point is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the default size for body text (assuming that the body font is 100%), but authors would need to check this for the particular fonts in use. When fonts are defined in relative units, the actual point size is calculated by the user agent for display. The point size should be obtained from the user agent, or calculated based on font metrics as the user agent does, when evaluating this success criterion. Users who have low vision would be responsible for choosing appropriate settings.

This Success Criterion and its definitions use the terms "contrast ratio" and "relative luminance" rather than "luminance" to reflect the fact that Web content does not emit light itself. The contrast ratio gives a measure of the relative luminance that would result when displayed. (Because it is a ratio, it is dimensionless.)

Note 5: The 18 and 14 point sizes for roman texts are taken from the minimum size for large print (14pt) and the larger standard font size (18pt). For other fonts such as CJK languages, the "equivalent" sizes would be the minimum large print size used for those languages and the next larger standard large print size.

Conversion from nonlinear to linear RGB values is based on IEC/4WD 61966-2-1 [IEC-4WD] and on "A Standard Default Color Space for the Internet - sRGB" [sRGB].

Ring-shaped light that passes through the aperture is focused by the condenser onto the biological specimen or material sample to be observed. Portions of the ring-shaped light are diffracted or scattered by structures of the specimen or features of the sample. The diffracted light enters the objective. In contrast, the portion of the ring-shaped light that passes directly through the specimen non-deviated or is reflected by the sample without scattering will not be collected by the objective. The light scattered by the specimen’s structures or sample’s features will appear brighter than the background areas of the specimen or sample where there is no light scattering.

G145: Ensuring that a contrast ratio of at least 3:1 exists between text (and images of text) and background behind the text

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The previously-mentioned contrast requirements for text also apply to images of text (text that has been rendered into pixels and then stored in an image format) as stated in Success Criterion 1.4.3.

Note 4: Background color is the specified color of content over which the text is to be rendered in normal usage. It is a failure if no background color is specified when the text color is specified, because the user's default background color is unknown and cannot be evaluated for sufficient contrast. For the same reason, it is a failure if no text color is specified when a background color is specified.

Leica darkfield microscopes are useful for the study of cells or tissues concerning a variety of life-science and forensic applications. Darkfield microscopy can also be helpful for material and earth-science applications.

Leica microscopes providing darkfield are commonly used in life science research for the visualization, analysis, and documentation of biological structures and cellular processes.

The contrast ratio of 7:1 was chosen for level AAA because it compensated for the loss in contrast sensitivity usually experienced by users with vision loss equivalent to approximately 20/80 vision. People with more than this degree of vision loss usually use assistive technologies to access their content (and the assistive technologies usually have contrast enhancing, as well as magnification capability built into them). The 7:1 level therefore generally provides compensation for the loss in contrast sensitivity experienced by users with low vision who do not use assistive technology and provides contrast enhancement for color deficiency as well.

G18: Ensuring that a contrast ratio of at least 4.5:1 exists between text (and images of text) and background behind the text

A contrast ratio of 3:1 is the minimum level recommended by [ISO-9241-3] and [ANSI-HFES-100-1988] for standard text and vision. The 4.5:1 ratio is used in this provision to account for the loss in contrast that results from moderately low visual acuity, congenital or acquired color deficiencies, or the loss of contrast sensitivity that typically accompanies aging.

Note 2: Font size is the size when the content is delivered. It does not include resizing that may be done by a user.

Note 1: Fonts with extraordinarily thin strokes or unusual features and characteristics that reduce the familiarity of their letter forms are harder to read, especially at lower contrast levels.

Note 4: When using text without specifying the font size, the smallest font size used on major browsers for unspecified text would be a reasonable size to assume for the font. If a level 1 heading is rendered in 14pt bold or higher on major browsers, then it would be reasonable to assume it is large text. Relative scaling can be calculated from the default sizes in a similar fashion.