Is an Ultrasonic Sensor or Infrared Sensor better for measuring distance (to/from a human body)? I'm trying to make a sensor to detect that a person is within 50cm to trigger a function. But when I use the HR-SR04 sensor, it works accurately on flat surfaces (e.g. walls, books) with a margin of error around 5cm, but not on uneven surfaces (e.g. humans) where the value randomly jumps between 0cm to 1000cm.
I was thinking whether using an IR distance sensor would be better.
That depends on your requirements. An IR distance sensor will work better with different materials, since IR light is mostly reflected in very broad angles (expect for very reflective materials, like mirrors), so that uneven surfaces are also detected. But you will get a significantly worse error margin in your 50cm range. Normal IR distance sensors are just an LED and an IR photo transistor, sometime with some extra components. So the LED is lighting up and the photo transistor is measuring, how much of the IR light is coming back. The IR sensor depends also on the reflectiveness of the materials and the level of surounding IR light.
But as you don't really need to measure the distance, but just want to know, if there is an object in the range, an IR sensor might be enough. You would need to change the threshold of your IR sensor according to your needs (by experimenting with it).
All in all I think, yes, an IR sensor can give you better results for your application, though not for really measuring the distance, but only for the presence in a range with rather big error margin.