How far, and how wide

Where the 26 metres comes from

Vishay quotes a transmission distance of 26 m for the TSOP18138. It is a real measurement, taken in the dark with a TSAL6200 LED at 50 mA pointed straight at it. The TK16's LED runs at 24 mA from 5 V and 13 mA from 3.3 V, and your room is not dark. The datasheet number is a ceiling, not your range.

Two beams, and the narrow one is the sender

The receiver's half angle is ±45°: 45° off straight on, it reaches half as far. The sender's beam is 40° in all, ±20°. So the sender is the half you aim, though intuition says the opposite because the receiver has the chip.

Both boards look out of their parts side, the receiver through its shield's window and the LED through its lens. Stand them face to face, parts side to parts side. At four metres, ±20° covers a patch about three metres across.

What daylight costs

The receiver ignores steady light, but it cannot pick a small signal out of a very large background. Vishay plots the signal it needs against the ambient infrared: flat in a dark room, rising steeply past a few watts per square metre. In direct sun it needs roughly thirty times as much, which is most of your range gone.

Nothing in software changes that. Shade the receiver, turn it away from the window, or put it behind a dark infrared-passing window, the kind on a television's front panel.

What buys you range

In the order they are worth it:

1. The sender's VCC on 5 V: about a third more distance, one wire.
2. Aim: a couple of degrees of tilt on a breadboard is a lot at the far end.
3. Shade on the receiver: worth more than everything else by a window.
4. A pale ceiling: bounced infrared is how remotes work when nobody aims.
5. Not too close: under about 20 cm the receiver is swamped and decoding fails on a perfect link.

A number to design to

For a project rather than a measurement: two to five metres indoors, within about 20° of straight on, with the sender on 5 V. That is a working figure from the arithmetic above, not a measurement of this kit; your room decides the rest.

One more line of the datasheet is worth knowing. Vishay's table of suitable codes is written for short bursts and points codes with bursts longer than ten carrier cycles, NEC among them, to a sister family, the TSOP182. This part's setting is the least strict of the three Vishay offers and it decodes NEC, as the remote in the box shows; if range in an electrically noisy room ever matters more than anything, that sister part is the one Vishay built for it.