Sensitivity vs Threshold

Sensitivity vs Threshold 

In video surveillance, sensitivity determines the smallest motion a camera can detect, while threshold defines the magnitude of that motion needed to trigger an event. A higher sensitivity setting allows the camera to register even minor movements, whereas a higher threshold makes it harder to trigger an event, requiring a more significant amount of motion to activate the motion detection. You often adjust these together to reduce false alarms from things like light changes while still capturing real events like a person or vehicle.

Sensitivity measures how responsive a system is to a stimulus, while the threshold is the minimum stimulus energy or change required to trigger a response, with sensitivity being the reciprocal of the threshold. In simpler terms, a high sensitivity means a low threshold and the system responds to small or subtle changes, whereas a low sensitivity implies a high threshold, requiring a significant change to elicit a reaction. 

Sensitivity

·        Definition: The ability of a system or person to detect a stimulus or respond to a change.

·        What it is: The ease with which a camera detects motion. 

·        Measurement: It is often defined as the inverse of the threshold. 

·        Function:  A high sensitivity means a system is more responsive and can detect very weak or small signals/changes.

·        How it works: A higher sensitivity value means the camera's motion detection algorithm is more "tuned in" to changes in pixels, allowing it to pick up on smaller or fainter movements.

·        Effect: A high sensitivity can lead to more alerts, as it's more likely to trigger from small, irrelevant movements. 

·        Example: A person with high sensitivity to noise might hear a faint sound that others don't. 

Threshold

·        Definition: The minimum level of stimulus energy, intensity, or change that is needed to activate a system or produce a detectable response.

·        What it is: The minimum amount of movement required to trigger a motion event.

·        Measurement: The specific level of input that is just enough to cause a reaction. 

·        Function: A low threshold means the system is easily triggered, while a high threshold requires a greater stimulus for a response.

·        How it works: It sets the bar for how much of a motion pixel change must occur before an alert is generated.

·        Effect: A higher threshold makes the system less likely to trigger, while a lower threshold will trigger the event more easily.

·        Example: A security camera's motion threshold determines the minimum amount of pixel change required to trigger an alarm. 

Relationship in Practice

·        Sensitivity and Threshold are Inversely Related: 

When sensitivity is high, the threshold is low, meaning less stimulus is needed for a response. Conversely, when sensitivity is low, the threshold is high, requiring a stronger stimulus to get a reaction. 

·        Adjusting for Performance: 

In tasks like motion detection, you adjust these settings together.

ü  Low sensitivity (high threshold): Reduces false alarms but may miss actual events. 

ü  High sensitivity (low threshold): Detects more subtle changes but increases the risk of false alerts.

Balancing Sensitivity and Threshold

·        Reduce False Alarms: You can use a combination of a higher threshold and moderate sensitivity to avoid triggering events from non-threatening movements (like swaying branches). 

·        Capture Key Events: You might use high sensitivity with a higher threshold to ensure you don't miss real events while still filtering out minor disturbances. 

Threshold value

As you mentioned the threshold value is a number in % and is the amount of pixels that is covered by the object relative the total amount of pixels in the picture. Let’s say that a person cover 15% of the total amount of pixels. Then the threshold value is 15%. For the system to detect that person the threshold level must be set to a lower value than 15%, let´s say 10%. Now, if something covers more than 10% of the pixels in the field of view the system will acknowledge that as an object of interest. But, that information will solely not trigger the system for motion detection… Therefore, we need a second parameter called Sensibility value.

Sensibility value

Sensibility is a parameter connected to if the object is moving slowly or fast. If the object is moving at all or if it is moving slowly or fast is determined from more mathematical calculations during a certain time interval. Let´s say that at time=1 the calculation gives the first threshold value. A specific time later at time=2 the second calculation gives the next threshold value and after time=3 it gives the third threshold value. If all these threshold values are the same the object is not moving and the system will not generate a motion trigg to start a recording of a video. But, if it is a difference between the threshold values something is happening with the object and it gets interesting. If the differences between the threshold values are small the object is probably moving slow. If the difference is big the object is probably moving fast. Let´s say that the system calculates a sensibility value that is between 0 and 100. A value of 0 means no change in motion, completely still. A value of 100 means t that the object is moving very fast. In Netcam system the Sensibility can be set to; very high – normal – very low. If you want to detect an object that moves very slowly you need to set the Sensibility to very high. Small changes in the sensibility value should be recognized as important and tell the system that here is something important going on. If the object is moving fast the sensibility value is high and it will also be recognized by the system as important. Well, why not always set the Sensibility to very high because then we will never miss a moving object? That is the million dollar question. If you set the Sensibility to very high it will detect everything and you will have a lot of false alarms! If you set the Sensibility to very low you will most likely never get a false alarm, but you will never get the moving object of interest either. Setting the Sensibility correct needs quite often some testing since it is very dependent of the situation.

‘Sensitivity’ is specified by most camera manufacturers using the ISO 12232 methodology. When using this saturation-based method, a higher ISO value means that it takes less light for the image to reach saturation but does not define how sensitive a sensor is to light (i.e., how many electrons are generated per incident photon) relative to the sensor noise.

ISO 12232 was established to give people an idea how a digital sensor compared to film speed, however it was never intended to give a full range of sensor performance. High-speed cameras are used in applications that demand good performance in low-light environments, which can not be determined by the ISO spec. To achieve a higher ISO rating the display settings of an image can be manipulated, such as by reducing the bit depth or full-well capacity. These specific changes make the image appear brighter but have an adverse effect on image quality and performance.

Consider the following limitations with the ISO 12232 specification when using it to compare high-speed cameras:

·        ISO 12232 does not account for noise performance. Instead, meaningful sensor qualities like Temporal Dark Noise (aka Read Noise) and Absolute Sensitivity Threshold, which indicates how well the sensor can identify detail from noise, are key attributes to discerning low light performance of the camera.

·        ISO is easily manipulated with added gain, which lowers Signal-to-Noise Ratio (SNR) and lowers Dynamic Range (DR). This trade-off gets masked because SNR and DR are not always reported, particularly not in relation to ISO. 

·        The rounding factor – Because ISO is based on film speed, manufacturers are instructed to round up to the closest defined ISO value above what was measured. This can inflate the value by up to 1/3 F-stop and is one reason it is not possible to get accurate photon level measurements with ISO as the basis for sensitivity.

·        The light source used for measuring ISO can be Tungsten or Daylight, and a monochrome camera using a Tungsten source will have a much higher spec when an IR filter is not used. Many applications do not have scattered or reflected light matching the spectrum of tungsten or daylight sources. Instead, it is best practice to use Spectral Responsivity plots (or QE curves) to determine how many electrons are generated relative to the number of incident photons across the visible and NIR spectrum. Spectral response curves are provided for all Phantom cameras.

Ultimately, there are too many unknowns to rely on the ISO 12232 specification when comparing sensitivity or any aspect of image quality. Vision Research has moved away from ISO as the way to spec sensitivity in favor of the EMVA 1288 standard, providing customers a full set of sensor parameters to evaluate the camera’s imaging performance.