The optotypes fields allow you to select both the letters to be displayed, and the order of letters on each line. You cannot change the array nor order of the lines displayed here. Custom Optotypes and Alphabets can be installed.

• Optotype: Select from the standard optotypes and those you have installed yourself.
• Alphabet: The available options are dependent on the OPTOTYPE selected above. SnellenU and SloanU use a wider selection of letters, but letters may not be of equal readability. BS4724.3 and ETDRS are standard alphabets using fewer letters, but with equal readability. Letters can be shuffled to prevent learning - see below

• Notation: Sets the notation type on the VA label on each chart line. Each notation type has a fixed array of chart lines. "Metres" is standard Snellen notation eg. 6/6; "Feet" is the US equivalent eg. 20/20; DecimalV is a decimal equivalent of Snellen notation eg. 6/60 = 0.1, 6/12 = 0.5; logMAR is explained here https://en.wikipedia.org/wiki/LogMAR_chart.
• Numerator: Sets the numerator of only the fraction notation types. If Notation is set to logMar or DecimalV, this setting makes no difference. "Actual" is the value entered in Distance (see below) rounded to the nearest 0.1m or 1ft, and VA labels will show this rounded numerator and a proportionally adjusted denominator. "Standard" is either 6 or 20 depending whether Notation is set to Metres or Feet above. VA labels will display as either 6/ or 20/ regardless of the value in Distance. Please note that it is not advisable to use Standard if the Distance varies a large amount from 6000mm(20ft), say outside 5000-7000mm(16-23ft). The numerator should be the tested distance or near to it.

The two CALIBRATION values are the ONLY values that influence the display size of the chart lines. The RECORDING values above only change the VA labels on the chart lines, and have no bearing on the chart lines themselves.

• Distance: Enter the distance from the patient's eye to the letter chart in mm. Eg 6m is 6000mm. You can work in feet (see Notation above) but you MUST enter the Distance in mm. If using a mirror to double the test distance, measure BOTH from the patient's eye to the mirror AND from the mirror to the chart and ADD the two distances (and don't forget to check the Mirrored/Direct setting - see below). If you are using Notation value of "Feet", Distance will be converted to the nearest foot, or to 20 depending on the value in Numerator.
• Length of the line below: Calibrates the chart for the pixel density of the display being used. Measure the ruler line as accurately as possible. Measure two more times, and if the values vary, enter an average. You will need to re-check this value if you change display hardware, graphics adapter settings, or if you turn your display from landscape to portrait or vice versa. Take particular care here if you are using extended display from a laptop as pixel density may be different between the laptop screen and the external display.

A maximum of 5 letters is displayed on each line. If the browser window is not large enough to display all 5 letters, fewer will be displayed. To measure the DISPLAY WIDTH, use a ruler to measure the horizontal width of your browser window in mm. Once you have applied the settings, you can vary this DISPLAY WIDTH value to improve the appearance and centration of the chart. However be aware that a smaller value may limit the number of letters that can be displayed on larger lines. You can use the SHUFFLE function (see below) to present the 5 letters required for a repeatable result if 5 letters won't fit.

• Sort: Unless the display is very large (about 1m wide by about 1.5m tall), the chart will display in pages - see SHORTCUTS below. Here you can select whether to display the page of the smallest or largest lines first on refresh or reconfigure.
• Mirrored/Direct: Select "Direct" if the chart is to be viewed directly, otherwise select "Mirrored" and the letters and lines will be reversed, but the labels not. If changing this setting, don't forget to set the Distance (see above) accordingly.
• Text colour: Select from the colour picker or enter any valid rgba string Eg. rgba(0,0,0,1) is black; rgba(0,0,0,0.1) is 10% black; rgba(255,0,0,1) is pure red.

Use these fields to specify keyboard shortcuts for operating the chart.

• Page Up/Page Dn: The chart displays upwards from the smallest chart line by default. If your display is large enough (about 1m wide by about 1.5m tall), all 10-14 lines will display, otherwise the chart will divide itself into pages and you will need to use Page Up or Dn to page through them.
  Paging can also be done with a mouse - the whitespaces above and below the chartlines are page up and down buttons.
• Shuffle/Unshuffle:: Shuffle randomises the letters normally shown on a chart line into a different order. You can shuffle as many times as you like. Unshuffle returns the letter order to the default.
• Duochrome:: Toggles a red/green background on any version of the chart.