Touchscreens

Display sizes

17"

19"

24"

The first touchscreens worked with a light barrier grid consisting of LED lights and infrared sensors, which was stretched in columns and rows in front of the monitor. The user's fingertip interrupts the light radiation and triggers a pulse in the corresponding row and column, from which the position of the fingertip follows in a simple manner. This technique achieves a resolution of around 5 mm. This resolution is sufficient for selecting larger ‘buttons’. Optical touchscreens are extremely robust, even against vandalism, so that they are still used today in cash and ticket machines.

Modern touchscreens can be divided into two main groups: resistive and capacitive. The two functional principles result in their respective advantages and disadvantages and their favoured applications.

Resistive vs. capacitive

Resistive Touchscreens are pressure sensors. When the screen is touched, two superimposed but invisible layers carrying electrical voltages are brought into contact with each other. The electrical (active) resistance (= resistance) collapses at the point of contact. The location of the contact point is determined using the ‘voltage divider rule’ known from circuit electronics.

Capacitive touchscreens have only a transparent metal oxide layer. An AC voltage source is applied to the corners and a tiny current flows through the finger when touched. In very simplified terms, you can imagine that the amount of current flowing out was stored in a capacitor, i.e. ‘capacitively’. The current intensity of the outflowing current depends on the distance of the point of contact from the four corners of the screen. The location of the point of contact is determined from the four measurements of the current strengths.

Resistive Touchscreens	Capacitive Touchscreens
Advantages: Can be operated with any stylus Can be operated with gloves and prostheses Also work with damaged surfaces Significantly lower (production) costs	Advantages: Operation with bare fingers or conductive input pens Practically wear-free glass surface Simultaneous recognition of touches (multi-touch), gesture recognition. Good image quality, high contrast Higher touch point accuracy More responsive
Disadvantages: Multi-touch and gesture operation only possible to a limited extent. Poorer legibility in sunlight. Wear due to mechanical stress during operation. Unwanted triggering possible through contact with objects.	Disadvantages: Interference from water and conductive materials is possible (electrostatic discharge: ESD effect). Electromagnetic interference (EMI effect) is possible Poor operability with standard gloves. People with dry skin or prosthetic hands are dependent on conductive styluses or special gloves. Risk of splintering if glass breaks, therefore unsuitable for food and pharmaceutical production
Typical applications: Resistive touchscreens are often used in environments with high demands on robustness: In industrially used panel PCs, e.g. for machine control In kiosk systems (e.g. trade fair information systems) In the automotive sector (navigation and multimedia systems) In-flight entertainment in commercial aircraft In office equipment (e.g. photocopiers)	Typical applications: Capacitive touchscreens are used in practically all communication and entertainment devices: Smartphones Tablets Media-Player (e.g. E-Books) Gaming consoles

Selection criteria

Professionally used touchscreens are expected to be robust, durable, have a long service life and be highly reliable. Therefore, when choosing a touchscreen, you must pay attention to

the appropriate IP protection class against dust and water,
the information on the operating temperature range and
the tolerance to shocks and vibrations.

Other aspects to consider when choosing the right touchscreen:

Screen size and resolution
Mounting options (wall, stand, swivel arm mounting)
Response time
Touch point accuracy
Brightness and contrast ratio for readability in different environments

Certifications and standards:

Industry standards (CE, FCC, RoHS, ...)
Special certifications for sectors such as the food and beverage industry
Specification for the medical field: including the DICOM specification for greyscale behaviour (contrast in black and white representations of medical imaging systems adapted to external lighting conditions).