vainternal: Internal Visual Acuity Functions
In JWiley/JWileymisc: Miscellaneous Utilities and Functions
vainternal R Documentation
Internal Visual Acuity Functions
Description
This function is one of several designed to help convert measures of
visual acuity recorded typically recorded as Snellen fractions
(e.g., 20/20 sees at 20 feet what is "typically" seen at 20 feet.
20/40 sees at 20 feet what is "typically" seen at 40 feet, etc.)
into statistically usable data. It can also parse text data
for Counting Fingers (CF) and Hand Motion (HM) and convert them to
approximate logMAR values.
This is an internal function and is not meant to be
called directly.
Usage
logmar(x, snell.numerator = 20, inverse = FALSE)
CFHM(x, zero)
snellen(snellenvalue, chart.values, chart.nletters)
Arguments
x
Character data of the form: “CF 10”, “HM 12”,
“HM”, “CF”, “CF 2”, etc. to be converted to
logMAR values.
snell.numerator
The numerator of a Snellen fraction.
It defaults to 20 (the most common one).
inverse
inverse = FALSE by default. If TRUE,
logmar will assume x is a logMAR value, and
calculate the denominator of a Snellen fraction using the
snell.numerator as the numerator.
zero
A “zero” logMAR value to be used for any CF or HM
value missing a number. May be an actual number or simply NA
snellenvalue
The observed snellen values
chart.values
The chart snellen values
chart.nletters
The number of letters per chart line
Details
This treats CF as approximately 200 letters (per Holladay), so
CF at 10 feet has Snellen value "equivalent" of 10/200. HM is
approximately 10 times worse, so HM at 10 feet approximately
10/2000. After conversion, rough equivalents are passed to
logmar to actually be converted.
Other functions are responsible for suitably parsing the text and
passing numbers to logmar. If inverse = FALSE (the default),
logmar calculates -log_{10}(\frac{snell.numerator}{x}).
If TRUE, then \frac{snell.numerator}{10^{-x}}.
The zero argument is used to specify a "zero" logMAR value.
In particular, this is used when no distance information is given
(e.g., only "HM" or "CF" as opposed to "CF 6" or "HM 4").
For the reasioning and rational behind this, see the "Details"
section of VAConverter.
For the snellen function, the input should be character data
and have a numerator and denominator separated by '/'. E.g.,
“20/20”, “20/40 + 3”. This handles both simple
and Snellen values that need to be interpolated given the
appropriate chart.
logMAR calculations including interpolating partial lines.
Given “20/25 + 3”, calculate the logMAR of 20/25 and
20/20 (the next step up since '+'), and go 3/chart.nletters
of the way between these two values. Similarly if
“20/20 - 3”, it will go partway between 20/25 and 20/30.
Note that it depends on the lines and letters per line
on the actual chart used. The chart.values and
chart.nletters should contain all the lines and number
of letters for the chart that was used.
These functions were written to deal
with a very specific style of recording visual acuity for a study I
worked on. It may or may not have much use elsewhere. CFHM
was not intended to typically be called by the user directly.
Generally, a higher level function, (e.g., VAConverter) would
be called.
References
Jack T. Holladay (2004). Visual acuity measurements.
Journal of Cataract & Refractive Surgery, 30(2),
pp. 287–290. \Sexpr[results=rd]{tools:::Rd_expr_doi("10.1016/j.jcrs.2004.01.014")}
See Also
VAConverter the overall function typically
called
Examples
## logMAR value for "perfect" 20/20 vision
JWileymisc:::logmar(x = 20)
## Go to and from logMAR value, should return "20"
## there may be slight error due to floating point arithmetic
JWileymisc:::logmar(x = JWileymisc:::logmar(x = 20), inverse = TRUE)
## logMAR value for 20/40 vision
JWileymisc:::logmar(40)
## logMAR approximations, note "HM" is just the zero value
JWileymisc:::CFHM(c("HM 20", "HM", "CF 20", "CF 12", "CF"), zero = 3)
## In cases where there is insufficient data, rather than choose
## an arbitrary value, you can may just use NA
JWileymisc:::CFHM(c("HM 20", "HM", "CF 20", "CF 12", "CF"), zero = NA)
JWiley/JWileymisc documentation built on Feb. 15, 2024, 12:23 p.m.
R Package Documentation
Browse R Packages
We want your feedback!
Note that we can't provide technical support on individual packages. You should contact the package authors for that.
| vainternal | R Documentation |
Internal Visual Acuity Functions
Description
This function is one of several designed to help convert measures of visual acuity recorded typically recorded as Snellen fractions (e.g., 20/20 sees at 20 feet what is "typically" seen at 20 feet. 20/40 sees at 20 feet what is "typically" seen at 40 feet, etc.) into statistically usable data. It can also parse text data for Counting Fingers (CF) and Hand Motion (HM) and convert them to approximate logMAR values. This is an internal function and is not meant to be called directly.
Usage
logmar(x, snell.numerator = 20, inverse = FALSE)
CFHM(x, zero)
snellen(snellenvalue, chart.values, chart.nletters)
Arguments
x |
Character data of the form: “CF 10”, “HM 12”, “HM”, “CF”, “CF 2”, etc. to be converted to logMAR values. |
snell.numerator |
The numerator of a Snellen fraction. It defaults to 20 (the most common one). |
inverse |
|
zero |
A “zero” logMAR value to be used for any CF or HM
value missing a number. May be an actual number or simply |
snellenvalue |
The observed snellen values |
chart.values |
The chart snellen values |
chart.nletters |
The number of letters per chart line |
Details
This treats CF as approximately 200 letters (per Holladay), so
CF at 10 feet has Snellen value "equivalent" of 10/200. HM is
approximately 10 times worse, so HM at 10 feet approximately
10/2000. After conversion, rough equivalents are passed to
logmar to actually be converted.
Other functions are responsible for suitably parsing the text and
passing numbers to logmar. If inverse = FALSE (the default),
logmar calculates -log_{10}(\frac{snell.numerator}{x}).
If TRUE, then \frac{snell.numerator}{10^{-x}}.
The zero argument is used to specify a "zero" logMAR value.
In particular, this is used when no distance information is given
(e.g., only "HM" or "CF" as opposed to "CF 6" or "HM 4").
For the reasioning and rational behind this, see the "Details"
section of VAConverter.
For the snellen function, the input should be character data
and have a numerator and denominator separated by '/'. E.g.,
“20/20”, “20/40 + 3”. This handles both simple
and Snellen values that need to be interpolated given the
appropriate chart.
logMAR calculations including interpolating partial lines.
Given “20/25 + 3”, calculate the logMAR of 20/25 and
20/20 (the next step up since '+'), and go 3/chart.nletters
of the way between these two values. Similarly if
“20/20 - 3”, it will go partway between 20/25 and 20/30.
Note that it depends on the lines and letters per line
on the actual chart used. The chart.values and
chart.nletters should contain all the lines and number
of letters for the chart that was used.
These functions were written to deal
with a very specific style of recording visual acuity for a study I
worked on. It may or may not have much use elsewhere. CFHM
was not intended to typically be called by the user directly.
Generally, a higher level function, (e.g., VAConverter) would
be called.
References
Jack T. Holladay (2004). Visual acuity measurements. Journal of Cataract & Refractive Surgery, 30(2), pp. 287–290. \Sexpr[results=rd]{tools:::Rd_expr_doi("10.1016/j.jcrs.2004.01.014")}
See Also
VAConverter the overall function typically
called
Examples
## logMAR value for "perfect" 20/20 vision
JWileymisc:::logmar(x = 20)
## Go to and from logMAR value, should return "20"
## there may be slight error due to floating point arithmetic
JWileymisc:::logmar(x = JWileymisc:::logmar(x = 20), inverse = TRUE)
## logMAR value for 20/40 vision
JWileymisc:::logmar(40)
## logMAR approximations, note "HM" is just the zero value
JWileymisc:::CFHM(c("HM 20", "HM", "CF 20", "CF 12", "CF"), zero = 3)
## In cases where there is insufficient data, rather than choose
## an arbitrary value, you can may just use NA
JWileymisc:::CFHM(c("HM 20", "HM", "CF 20", "CF 12", "CF"), zero = NA)
R Package Documentation
Browse R Packages
We want your feedback!
Note that we can't provide technical support on individual packages. You should contact the package authors for that.
Embedding an R snippet on your website
Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.