5  EP 3 800 002 A1 6  of the shield. [0028] In an embodiment, the optics assembly is con- figured toatleastpartly,preferablysubstantially,conform the at least one laser beam to the at least one beam passage (viewed in a cross-section that is normal to an optical axis of the beam). [0029] Thus, a substantially close fit may be provided between beam passage andthe laser beam at the beam passage (viewed in a cross-section that is normal to an optical axis of the beam), so that the laser beam may substantially pass through the beam passage. [0030] In an embodiment, the at least one beam pas- sage includes or is configured to provide a constriction in a fluid flow path through the beam passage, wherein in particular the beam passage includes and/or is part of a convergent-divergent nozzle, in particular a de-Laval type nozzle. [0031] Such a constriction can enable that a fluid flow speed in the fluid flow path is increased, in particul ar downstream of the constriction. An increased fluid flow speed can provide improved repulsion ofby-productsout of and/or away from the beam passage. [0032] The optics assembly can be configured in var- ious ways,and can include one ormore opticalelements (e.g.being separateopticalelements,oropticalelements being integrated with each other, or a single optical ele- ment providing different beam shaping functions). [0033] Preferably, the optics assembly comprise s, preferably for each laser beam of the at least one laser beam: [0034] In this way, the laser beam can be focused on the beam passage, while the laser beam can also be focused on the treatment area, i.e. spaced away from the beam passage. [0035] The main focal length of the second optical unit is preferably substantially large compared to the main focal length of the first optical unit, so that a distance between the respective fociofthe firstand second optical units can be large compared to a distance between the first and second optical units. In this way, the shield can be spaced awayfrom the treatmentarea while the optics assembly can be relatively compact. [0036] In an embodiment, at least one, preferably each, of the first and second optical units is configured to produce a line focus. To that aim e.g. the optical unit can include a substantially cylindrical lens or cylindrical mirror. [0037] Such a configuration enables that the laser beam can be focused at each of two different focal planes, e.g. without any optical elements in between the two different focal planes, in particular when the line foci are oriented substantially perpendicular to each other (i.e. at an angle of 90 degrees). [0038] In an embodiment, the system is configured to emitgas,e.g.a jetofgas,preferably towardsa treatment area,e.g.to remove liquid,e.g.rain water,fromthe treat- ment area. For example, the gas can be emitted via the laser beam passage of the shield. [0039] Effectiveness and/or efficiency of the beam de- livery can be improved, wherein the emitted gas can dry the treatmentarea byremovalofliquid.Such liquid (when present) could have otherwise been subject to the laser treatmentprocess,thustaking up lasertreatmentenergy. In other words, by removing liquid in this way, the beam deliveryforthe laser treatmentofany remaining material can be more effective and/or efficient. [0040] In an embodiment, the system includes acover means thatisconfigured tosubstantiallyclose offorcover a space which includes a treatment area. For example, during use, a vent can be provided, e.g. between the covermeans andan objectto be treated,forventing gas, e.g. air, from said substantially closed-off space. [0041] Such cover means can contribute tolasersafety by blocking and/or absorbing laser light, e.g. scattered laser light. Also, the cover means can be configured to provide a vent, e.g. to enables that liquid (if any) can be removed by a flow of gas (ratherthan e.g.being retained in the substantially closed off space). [0042] In an embodiment, the or each beam passage, in particular a respective relatively narrow width thereof, is dimensioned to conform to a respective beam width of the or each respective laser beam, in particular a beam width in the rangefromthe 1/e $^{2}$ beamwidth to the 99.99% intensity beam width. [0043] Ithas been found thatsuch a configurationpro- vides an advantageous trade-off between allowing a large portion of the laser beam to pass through the pas- sage on the one hand and protection againstby-products on the other hand. [0044] In an embodiment, the system includes a plu- rality of, e.g. two, beam passages which are associated with a respective plurality of laser beams. [0045] The plurality oflaserbeamsand passages may be arranged such that respective treatment areas of the laser beams complement each other, e.g. by together formingalargertreatmentarea orsetoftreatmentareas. In this way, laser treatment processes may be sped up and/or larger objects may be treated. [0046] In an embodiment, the system is configured for treating, in particular cleaning, a surface, in particular a 4 - a first optical unit, e.g. a lens or a mirror or a set of lenses and/or mirrors, for focusing the laser beam with respect to a first spatial direction which is sub- stantially transverse (orthogonal)toa secondspatial direction, wherein a main focal length of the first op- tical unit substantially corresponds to a distance be- tween the first optical unit and the respective beam passage; and - a second optical unit, e.g. a lens or a mirror or a set oflenses and/ormirrors,forfocusing the laserbeam with respect to the second spatial direction, wherein a main focal length of the second optical unit sub- stantially corresponds toa distance between thesec- ond optical unit and a treatment area.