On most oscilloscopes, there is a blank space that moves along as the data is sampled. Is there a name or phrase that described what that thing is? When you put the oscilloscope on a lower time/division, you can see it moving across the screen. In case I'm unclear, here's a picture where the waveform is not completed and the scope is in the middle of capturing:

Oscilloscope gathering waveform

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    \$\begingroup\$ A name? It's a time axis portion corresponding to the future. Call it "future". \$\endgroup\$
    – Eugene Sh.
    Aug 9, 2016 at 19:48
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    \$\begingroup\$ The blank space doesn't move, and it is of no interest so it doesn't have (or need) name. The lighted part is called the trace, and moves. \$\endgroup\$
    – JRE
    Aug 9, 2016 at 20:14
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    \$\begingroup\$ I'd use "the sweep" to refer to what I think you're trying to refer to, though that really just refers to the action of the beam moving left to right across the screen. The "blank space" is the oldest part of the sweep, where the phosphor glow has decayed. \$\endgroup\$ Aug 9, 2016 at 20:22

2 Answers 2


I guess you're referring to the horizontal axis, which represents time.

First of all there is a trigger which starts data capture. As data is received, it is sampled on the display in the shape of the waveform. As you say, "the waveform is not completed and the scope is in the middle of capturing" and the right-most point of the waveform is the current signal level. The blank space on the right will be completed as... time passes and data is captured and displayed.

This is visible when you play with horizontal sensitivity to decrease sweep speed (a smaller piece of signal on the same display width).

Read here about sweep speed: http://www.facstaff.bucknell.edu/mastascu/elessonshtml/measurements/scope1.htm


Unlike a scrolling display possible on digital scopes the analog CRT scope draws from left to right. The scope trigger circuit determines when the trace starts.

enter image description here

Figure 1. The electron beam is deflected by the charge on the horizontal and vertical deflection plates. A sawtooth wave applied to the horizontal plates to give a linear left to right trace. Image source: hit.bme.hu.

The phosphor glows when struck by the electron beam and then decays. The phosphor is chosen to give a reasonable balance between persistance at low trace speeds (where a long decay would be advantages) and quick decay to prevent blur on changing waveforms at high speed. This could be compensated for somewhat by adjusting the beam intensity.

Is there a name for that moving blank space on an oscilloscope?

I like @EugeneSh's comment that the blank space is "the future". However, there is a ghostly line visible on the right half of the screen and this is the past (obviously from the previous trace).

  • \$\begingroup\$ This is a good answer, but digital scopes have a moving beam or whatever as well. \$\endgroup\$
    – Snoop
    Aug 10, 2016 at 11:26
  • \$\begingroup\$ Of course. That's why I said, "Unlike a scrolling display possible on digital scopes ...". \$\endgroup\$
    – Transistor
    Aug 10, 2016 at 12:43

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