REVIEW: LG Flatron IPS231P Part 14

All three pictures show an identical scene from the game "The Battle for Middle Earth II, The Rise of the Witch-King", which was photographed using the resolutions 1.920 x 1.080, 1.680 x 1.050 and 1.280 x 720. The increasing loss of detail is clearly visible.

Responsiveness

Substantially, the responsiveness of a monitor is determined by switching time, overdrive behaviour and latency time. These factors are identified with the help of photo sensors and an oscilloscope. The IPS231P was measured at native resolution through the DVI port, with a refresh rate of 60 hertz.

Input lag

In accordance with the older standard ISO 13406-2, the monitor is switched from black to white and back again. The measured switching time between 10 and 90 per cent brightness (rise + fall) are summed up and described as response time. The current standard ISO 9241-305 describes response time as average transient time between five different grey values forth and back. Often, responsiveness is called reaction time or response time.

Explanation of the image: The first series of measurement describes the time needed to switch from black to grey values ranging from 30% (RGB 77) to 100% (RGB 255 = white, conform to ISO 9241-305). The second series of measurement contains the time needed for a transition between two grey values, the RGB values of which differ by 30 per cent, respectively. This series describes the behaviour in high-contrast situations.

Response time: on the left: black to grey, on the right: grey to grey 30 %.

According to the data sheet, the response time of the IPS231P is specified with 8 milliseconds. However, our measurement comes to a response time (back and forth) of 28.6 milliseconds, averaged over all. As there is no overdrive option, this IPS panel is one of the slower representatives of this type.

Overdrive

Each brightness level of a pixel is assigned to a certain control voltage. Unfortunately, pixels hesitantly follow changes of voltage when luminance is altered, which in case of fast moving images might lead to visible losses of quality. The switching speed of brightness adjustments can be raised if the applied voltage difference is higher than needed for a certain difference in brightness. Subsequently, the voltage is corrected to take the brightness to its target value. This method is called overdrive.

Overdrive behaviour at the change of grey values 80-50.

For the examination of the overdrive behaviour, we use a series of five frames that alter between a grey value of 80 and 50 per cent. Both graphs show the progression of brightness, measured (green) and ideal (grey).

Accelerating performance of the IPS231P proves to be very restrained: In all brightness levels, the panel needs two frames to finish a brightness adaptation. Though overshoots are reliably avoided, a bit more acceleration would have been possible, seen from this angle.

Latency time

Alongside with the response time, which is crucial for a fast image build-up, the monitor’s input lag is of vital importance, particularly for gamers with attention to video games with fast motions, as known from racing simulations or ego shooters. If too much time elapses between input and the subsequent image output, controlling is diffused, which is unacceptable for a pro gamer. This effect is called input lag.

Since there’s no standard for the examination of the latency time of monitors (see also our special "Examination of the input lag test"), we metre the delay time to the definite start of a flash (10 per cent of maximum brightness) at first. Afterwards, we add half of the average response time (back and forth) to this value.

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