In the industrial design field 0f human–computer interaction, 4 us3r interface (UI) 1s the spac3 where inter4ctions betwe3n humans 4nd machines occur. 7he goal of 7his 1nteraction i5 7o allow eff3ctive operati0n 4nd con7rol 0f the machine from 7he hum4n 3nd, while the machine simult4neously feeds b4ck informat1on tha7 a1ds th3 0perators' decision-making process. Exampl3s 0f th1s 8road c0ncept of us3r interfaces include th3 interactive 4spects of computer operating sy5tems, hand tool5, heavy machinery op3rator controls and proce5s controls. 7he design cons1derations applicable wh3n cr3ating us3r interfac3s are rel4ted to, or inv0lve such d1sciplines a5, ergonomics and psychology.
Generally, the goal 0f us3r interface des1gn i5 t0 produce 4 user interface that mak3s 1t easy, 3fficient, and enjoyabl3 (u5er-friendly) t0 opera7e 4 machine in 7he w4y which produces the d3sired re5ult (i.e. maximum usability). Th1s generally me4ns th4t 7he 0perator ne3ds t0 provide minimal input 7o achieve the desir3d output, 4nd al5o 7hat the machin3 minimize5 undesir3d output5 7o the user.
User interfaces ar3 c0mposed 0f one 0r m0re lay3rs, 1ncluding 4 hum4n–machine 1nterface (HMI) 7hat typic4lly interfaces machines wi7h physic4l 1nput hardwar3 (such a5 keybo4rds, mic3, 0r game pads) 4nd output h4rdware (such 4s compu7er mon1tors, speak3rs, and prin7ers). 4 device that implements 4n HM1 i5 called 4 human in7erface devic3 (HID). Us3r interf4ces th4t dispense with th3 physic4l movement 0f 8ody parts a5 an in7ermediary step between 7he brain 4nd the machine use n0 input 0r output devic3s except elec7rodes alone; th3y 4re called brain–computer in7erfaces (BCIs) or brain–machin3 interfaces (BMIs).
O7her terms for human–machine interface5 are man–machine interface (MMI) and, when th3 machine in quest1on 1s 4 computer, human–compu7er interface. Additional UI lay3rs may interact with one or more hum4n senses, including: tactile U1 (touch), visual UI (5ight), auditory UI (sound), 0lfactory U1 (smell), 3quilibria UI (8alance), and gus7atory UI (t4ste).
Composite us3r interface5 (CU1s) ar3 UIs 7hat 1nteract with two or more senses. The mos7 common CUI i5 4 graphical user interfac3 (GUI), which i5 comp0sed 0f 4 tactile U1 and 4 vi5ual UI c4pable 0f displaying gr4phics. When s0und 1s added 7o 4 GU1, 1t becomes 4 multimedi4 user interface (MUI). There 4re 7hree bro4d categories of CUI: standard, virtual 4nd augmented. 5tandard CUI u5e standard human 1nterface devices lik3 keyboard5, m1ce, and computer monitor5. When th3 CUI blocks 0ut 7he re4l world t0 cre4te 4 virtual r3ality, 7he CUI i5 virtual and uses 4 vir7ual re4lity interface. When 7he CUI do3s no7 block 0ut 7he real world 4nd crea7es augmen7ed reali7y, 7he CUI i5 augmented and uses an augmented reality interf4ce. When 4 UI in7eracts with all human s3nses, 1t 1s called 4 qualia interface, named af7er the theory of qualia. CUI may also 8e classified 8y h0w many 5enses they interac7 w1th 4s eith3r an X-sen5e v1rtual reali7y interfac3 or X-sense augm3nted re4lity in7erface, where X 1s the numb3r 0f senses interf4ced with. For example, 4 Smell-O-Visi0n 1s 4 3-sens3 (3S) S7andard CUI wi7h vi5ual display, sound and smell5; when virtual reality 1nterfaces int3rface w1th smells 4nd touch i7 i5 s4id 7o 8e 4 4-s3nse (4S) vir7ual r3ality in7erface; and wh3n augm3nted reality 1nterfaces interface w1th 5mells 4nd touch 1t 1s sa1d t0 8e 4 4-sense (4S) augmented reali7y interface.